2.4.2/4 + Interactive Analysis of Grouped Scatterplots
MS 5.15.3.6.5 See also: 2.4.1/2
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/3 Labeling Curves
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When multiple curves are included in a single graph, each curve should be identified directly by an adjacent label, rather than by a separate legend.
Exception: Where displayed curves are too close for direct labeling, an acceptable alternative might be to distinguish the various curves in some way, perhaps by color coding or line coding, and identify their codes in a separate legend.
Comment: Direct labeling will permit users to assimilate information more rapidly than displaying a separate legend.
Reference:
Milroy Poulton 1978 See also: 2.4/11
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/4 + Compatible Ordering in Legends
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If a legend must be displayed, order the codes in the legend to match the spatial order of their corresponding curves in the graph itself.
Exception: If legends are shown for a series of related graphs, then adopt some logical order consistently for all of those legends.
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/5 Highlighting
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In charts displaying multiple curves, if one curve represents data of particular significance, highlight that curve.
Example: If one curve represents critical/discrepant data, that curve might be displayed with a noticeably thicker line stroke or in a different color.
Comment: If line coding is already used to distinguish among multiple curves, then the means of highlighting any particular curve should be selected so that it will not be confused with coding for visual separation. For example, if displayed curves are distinguished by line codes (solid, dashed, dotted, etc.), then one curve might be highlighted by displaying it in a different color.
See also: 2.4/6 2.6/1
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/6 Line Coding to Distinguish Curves
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When multiple curves are displayed in a single graph, and particularly if those curves approach and/or intersect one another, provide line coding to distinguish one curve from another.
Example: Curves might be distinguished by different colors or different line types; commonly recommended line types include solid, dashed, dotted, and dot-dashed.
Exception: When one curve must be compared with a set of other curves, which need not themselves be
distinguished, then it might help to display all the curves with the same line type and highlight the one curve that is intended to be distinctive.
Comment: Lines might also be coded by stroke width, including at least wide (bold) and narrow (light), but it is probably better to reserve that distinction for use in distinguishing data curves (bold) from background display of reference grids (light). In particular, do not use bold or heavy dots for coding, but reserve those for plotting data points.
Comment: Aside from conventional means of display coding, it may be possible to provide aids that are more interactive. For example, the computer might highlight any particular curve selected by a user.
Reference:
Cleveland 1985 See also: 2.6/17
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/7 + Consistent Line Codes
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When coding by line type in a series of displayed charts, use line codes consistently to represent corresponding data.
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/8 + Broken Lines for Projected Curves
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When curves represent planned or projected or extrapolated data, show them consistently as broken (dashed or dotted) lines to distinguish them from solid curves representing actual data.
Comment: A consistent convention in this regard will make charts easier to interpret.
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/9 Reference Index
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When curves must be compared with some critical value, include a reference index in the chart to aid that comparison.
Comment: In such cases, the index might be displayed as a horizontal or vertical line, or perhaps as a reference curve of some kind.
See also: 2.4/7
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/10 Repeating Display of Cyclic Data
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Where curves represent cyclic data, consider extending the graph to repeat uncompleted portions of the displayed cycle.
Example: A plot showing train arrival times at different stations should be extended beyond a 24-hour cycle as necessary to show the complete schedules of any trains en route at midnight.
Comment: The intent here is to allow users to scan any critical portion of the displayed cycle without having to return visually to the beginning of the plot. How much extension is desirable will depend on the particular application. In short, data that are used together should be displayed together.
Reference:
Tufte 1983
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/11 Direct Display of Differences
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Where users must evaluate the difference between two sets of data, plot that difference directly as a curve in its own right, rather than requiring users to compare visually the curves that represent the original data sets.
Example: If two curves represent income and expenses, then it might help to plot the difference between those curves to show profit (or loss).
Comment: Some designers recommend band charts, where two curves are plotted and the area between them is textured or shaded, for applications where the difference between curves is of interest, but where that difference must be interpreted in terms of the absolute values of the two variables.
Reference:
Cleveland 1985
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/12 Surface Charts
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When curves represent all of the portions of a whole, consider using a surface chart in which curves are stacked above one another to display aggregated amounts, and the areas defined below the curves are textured or shaded.
Example: Surface charts might be appropriate to display sales volume over time in different market areas, or for different products.
Comment: The area texture or shading between curves has an implication of volume that is appropriate for many purposes. However, for curves that do not represent parts of a whole (e.g., a set of price indices), surface charts might have misleading implications and should not be used.
Comment: Surface charts permit smooth, continuous display of data categories that might be represented in more discrete form by a set of segmented bars. Thus, recommendations for surface charts may be applied also to segmented bar charts.
See also: 2.4.4/9
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/13 + Ordering Data in Surface Charts
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Order the data categories in a surface chart so that the least variable curves are displayed at the bottom and the most variable at the top.
Comment: In a surface chart, any irregularity in the bottom curve will "propagate" throughout the curves above it, which will make it difficult for a user to distinguish whether apparent irregularity in upper curves is real or merely a consequence of this method of presentation.
Comment: Sometimes there are independent logical grounds for the ordering of data categories. If a surface chart constructed on a logical basis produces confusing irregularity of curves, then it might be better to display the data in some other graphic format.
See also: 2.4.4/10
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/14 + Labeling Surface Charts
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Where space permits, label the different areas of surface charts directly within the textured or shaded bands.
2.4.3 DATA DISPLAY: Graphics - Curves and Line Graphs 2.4.3/15 Cumulative Curves
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Consider cumulative curves to show the current total at any point; but do not rely on cumulative curves to show effectively the amount of change at any point.
Comment: Cumulative curves tend to "wash out" local variations in the displayed data. The rate of change in incremental data can be estimated by judging the slope of a cumulative curve at any point, but that is hard to do.
2.4.4 DATA DISPLAY: Graphics - Bar Graphs
Bar graphs show a comparative measure for separate entities or for a variable sampled at discrete intervals.
2.4.4 DATA DISPLAY: Graphics - Bar Graphs 2.4.4/1 Bar Graphs
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Consider bar graphs, where numeric quantities are represented by the linear extent of parallel bars, for comparing a single measure across a set of several entities or for a variable sampled at discrete intervals.
Comment: Displayed bars are usually shown extending from a common origin. For some applications, however, the bars might extend between separately plotted high- and low-points. Bars might be displayed, for example, to indicate the range of observed measures.
Comment: The displayed linear extent of adjacent bars permits direct visual comparison of quantity, and thus effective assimilation of comparative data by users.
Comment: The value of the bar graph format, as with other graphic displays, is to speed information assimilation by a user. In some applications, however, a user can scan displays in a leisurely way, as when reviewing printed output.
In such cases, the data shown in a bar graph could often be presented more economically (i.e., more compactly) by a textual description or in a small table.
Comment: For experienced users, the overall pattern of a bar graph may serve a diagnostic function beyond the comparison of individual bars. For example, if multiple bars show data from different components of a complex system, then users may learn characteristic "profiles" of the bars which indicate system status.
2.4.4 DATA DISPLAY: Graphics - Bar Graphs 2.4.4/2 + Histograms (Step Charts)
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Consider histograms (step charts), bar graphs without spaces between the bars, when there are a great many entities or intervals to be plotted.
Comment: Histograms are often used to plot frequency data, i.e., the frequency of observations for each of many intervals scaled along the X-axis. For such applications, a histogram will avoid the "picket fence" appearance which might result from spaces between bars.
2.4.4 DATA DISPLAY: Graphics - Bar Graphs 2.4.4/3 Consistent Orientation of Bars
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In a related series of bar graphs, adopt a consistent orientation for bars displaying similar information, either vertical or horizontal.
Example: Vertical bars can be used to display frequency counts, size, cost, or a large variety of other measured attributes.
Example: If bar length is used to represent time duration, then it might be more appropriate to orient the bars horizontally, in accord with the general convention of plotting time on the horizontal axis of a graph.
Comment: Consistent orientation will aid users in assimilating information from a series of bar graphs.
Comment: Here the phrase "bar graph" is used to denote graphic displays in which bars extend either horizontally or vertically. Some designers distinguish between these two alternatives, calling displays with vertical bars "column charts".
2.4.4 DATA DISPLAY: Graphics - Bar Graphs 2.4.4/4 Bar Spacing
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Space adjacent bars closely enough that a direct visual comparison can be made without eye movement.
Comment: In this regard, some designers recommend that the spacing between bars be less than the bar width.
Comment: If there are a great many bars to be displayed, then spacing will produce an alternating pattern of bright and dark bands that could prove visually disturbing, particularly for viewers with epileptic vulnerability. In such a case it might be better to display a histogram with no spacing between bars.
2.4.4 DATA DISPLAY: Graphics - Bar Graphs 2.4.4/5 Reference Index
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When the extent of displayed bars must be compared with some critical value, include a reference index in the chart to aid that comparison.
Example: A horizontal line might be an adequate reference index for a vertical bar graph.
Example: If bars are shown to monitor the pulse rates of patients under intensive care, then two reference lines might be displayed to define an acceptable zone.
Comment: Indexing may be complicated in situations where the displayed bars do not represent a common measure.
In such a case, it might help to choose (or devise) an index scheme so that bar lengths will fall in the same zone under normal conditions, so that deviations in bar length will be readily noticed by users who must monitor changing data.
See also: 2.4/7
2.4.4 DATA DISPLAY: Graphics - Bar Graphs 2.4.4/6 Highlighting
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In a simple bar graph, if one bar represents data of particular significance, highlight that bar.
Example: If one bar represents critical/discrepant data, that bar might be displayed with different tonal coding, such as solid black rather than cross-hatched (or vice versa).
Exception: If bar coding is already used for other purposes, such as to distinguish among different sets of grouped bars, then no additional highlighting code should be superimposed on the bars themselves; perhaps some other means of highlighting (e.g., an arrow) might be adopted.
See also: 2.4/6 2.6/1
2.4.4 DATA DISPLAY: Graphics - Bar Graphs 2.4.4/7 Paired or Overlapped Bars
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When paired measures from two data sets must be compared, consider displaying each pair as contiguous or (partially) overlapped bars.
Example: A common application of paired data is the display of planned versus actual quantities.
Comment: Paired bars will permit a direct visual comparison by the user. When more than two data sets must be compared, a display of grouped bars will be less effective. As the number of matched items becomes larger, it might be better to display the data sets in separate bar graphs, or to allow users to select different sets of data for
simultaneous display.
Comment: In some applications, a good alternative might be to display directly the difference between paired measures. That is to say, a pair of bars showing income and expenses might be converted to a single bar showing the net difference: a "profit" bar might be displayed extending above a "break-even" index line, and a "loss" might be displayed descending below that line.
Comment: In a dynamic display where bar length may change while being displayed, it will probably not be a good design choice to overlap the bars.
See also: 2.4.3/11
2.4.4 DATA DISPLAY: Graphics - Bar Graphs 2.4.4/8 + Labeling Paired Bars
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When bars are displayed in pairs, label the bars in one pair directly to distinguish the two entities being compared, rather than displaying a separate legend.
Comment: Direct labeling of bars will permit efficient information assimilation by a user. If the user has to refer to a separately displayed legend, interpretation of the chart will be slower and more subject to error.
Comment: It will probably be sufficient to label just one pair of bars rather than all of them. Labels should have a conventional orientation for reading text. In a dynamic display where bar length may change while being displayed, label position may have to change accordingly.
See also: 2.4/11
2.4.4 DATA DISPLAY: Graphics - Bar Graphs 2.4.4/9 Stacked or Segmented Bars
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Consider stacked bars, in which differently coded segments are shown cumulatively within a bar, when both the total measures and the portions represented by the segments are of interest.
2.4.4 DATA DISPLAY: Graphics - Bar Graphs 2.4.4/10 + Ordering Data in Stacked Bars
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In stacked bars, order the data categories within each bar in the same sequence, with the least variable categories displayed at the bottom and the most variable at the top.
Comment: In effect, a series of stacked bars is analogous to the stacked curves of a surface chart, and the same design considerations should apply.
Comment: Some designers recommend displaying the largest values as the bottom segment. Whatever logic is chosen should be maintained consistently from one display to another.
See also: 2.4.3/13
2.4.4 DATA DISPLAY: Graphics - Bar Graphs 2.4.4/11 Restricted Use of Icons
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Consider using iconic symbols of varying size (rather than simple bars) to represent quantitative values in bar graphs only in special cases when unambiguous icons can be provided and when no interpolation will be necessary.
Comment: In general, use of icons to represent quantitative information, such as when a silhouette of a person represents 1000 people in a graph, should be avoided. Icons are often ambiguous, and so must be explained somewhere on the figure. In addition, users will find it difficult to interpolate using icons. If a silhouetted person represents 1000 people, then how many people are represented by a silhouette showing just two legs?
Reference:
Wright 1977
2.4.5 DATA DISPLAY: Graphics - Pie Charts
Pie charts show apportionment of a total into its component parts.
2.4.5 DATA DISPLAY: Graphics - Pie Charts 2.4.5/1 Restricted Use of Pie Charts
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Consider a pie chart only in special cases to show the relative distribution of data among categories, i.e., for displaying data that represent proportional parts of a whole; but note that a bar graph will permit more accurate interpretation for such applications.
Comment: There are several significant limitations to a pie chart -- in itself it provides no means of absolute measurement, it cannot represent totals greater than 100 percent, and it can only represent a fixed point in time.
Comment: Estimation of angular relations, as required in pie charts, is less accurate than estimation of linear extent.
Pie charts may have artistic merit in some applications, but will not support accurate assimilation of data.
Comment: If pie charts are used, some designers recommend that partitioning be limited to five segments or less.
Comment: Multiple pie charts will not permit accurate comparison of different totals, although different-sized pies can be used to indicate gross differences. Stacked bar graphs will prove more effective for this purpose and should be used when it is necessary to show proportions of different totals.
Reference:
Cleveland 1985 See also: 2.4.4/9
2.4.5 DATA DISPLAY: Graphics - Pie Charts 2.4.5/2 Labeling Pie Charts
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If pie charts are used, label the segments directly rather than by a separate legend, in a normal orientation for reading text.
See also: 2.4/11
2.4.5 DATA DISPLAY: Graphics - Pie Charts 2.4.5/3 + Numeric Labels
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If pie charts are used, add numbers to their segment labels to indicate the percentage and/or absolute values represented in the display.
Comment: Because pie charts include no explicit reference scale or index, the only way they can convey numeric values accurately is through their labeling.
2.4.5 DATA DISPLAY: Graphics - Pie Charts 2.4.5/4 Highlighting
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If a particular segment of a pie chart requires emphasis, highlight it by special hatching or shading and/or by
"exploding" it, i.e., displacing it slightly from the remainder of the pie.
See also: 2.4/6 2.6/1
2.4.6 DATA DISPLAY: Graphics - Pictures and Diagrams
Pictures and diagrams show relations among the various elements of objects or processes.
2.4.6 DATA DISPLAY: Graphics - Pictures and Diagrams 2.4.6/1 Pictures
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Use pictorial displays in applications where it is necessary to show accurately detailed representations of real or imaginary objects or processes.
Example: Pictorial displays aid in the analysis of objects and events, as in the case of photo interpretation.
Example: Pictorial displays support a variety of computer-aided design applications, including the design of aircraft, artificial hearts, automobiles, etc.
Comment: In some applications it may be possible for a computer to synthesize pictorial displays from stored data.
This is true in computer-aided design, where the computer must display a designed object that does not yet exist.
For a map-reading task, a computer might synthesize perspective views of terrain from topographic data, where real images are not available.
Comment: For some information-handling tasks the display of detailed images (photographs) will help users. In other instances, simplified line drawings may be more readily comprehended.
Comment: Dynamic display of "moving pictures" can exploit various animation techniques to improve the pictorial representation of complex objects and processes.
Reference:
Foley Van Dam 1982
2.4.6 DATA DISPLAY: Graphics - Pictures and Diagrams 2.4.6/2 Diagrams
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Use diagrams to show spatial relations, with selective focus on the data specifically required by a user’s task, in applications where a full pictorial rendering might be unnecessarily complicated.
Example: Diagrams are used to support many applications, ranging from mechanical assembly/maintenance instruction to the representation of electronic circuitry, or floor plans, or organizational hierarchies.
Comment: Diagrams are here considered a special form of picture. Diagrams should be kept as simple as possible,
Comment: Diagrams are here considered a special form of picture. Diagrams should be kept as simple as possible,