Dual Axis Chart: Recommended Style and Tips

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Use a Dual Axis Chart:

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• To show correlation (or lack of correlation) between two data series;

• Use a bar chart when showing change over time, comparing values of different categories, or comparing parts of a whole;

• Use a line chart for changes over time to show trends, relationships, change, and predictions.

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Recommended Style 

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1. Bars: The gap width between bars should be as minimal as possible to avoid narrow or thin bars (see chart below).

2. Lines: Do not add symbols or data points to line segments, such as circles, squares, triangles.

3. Line Weight: Do not make lines too thick. 1 - 1.5 pt is sufficient.

4. Color: All bars in a single chart should be the same color and shade since they measure the same variable. Highlight the important bar/s mono chromatically using a different shade of the same color. When bars of a different color are needed, use contrasting color. See approved colors.

5. Base line: The base line of a dual axis chart should always start at zero. 

6. Bar Data: When the data is not time based, arrange data in the descending or ascending order. 

7. Values: When possible, remove excessive zeros from values.

8. Labeling: When appropriate, the X and Y axis should be clearly labeled. The Y axis label should be turned 90 degrees, reading from bottom to top (see chart below). The X axis should not be labeled at an angle. Font type: Calibri Light. Font size: 11.

9. Values: When possible, remove excessive zeros from values. 

10. Title: Begin your title with the portion mark classification, followed by Figure #: Title. Font type: Calibri Regular. Font size: 14.

11. Classification and Sourcing: Below the dual axis chart, bottom left, list the overall classification of the chart and directly below the classification, list the source and source stream/s. Font type: Calibri Light Italic. Font size: 9.

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Tips

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• Make sure the y-axes are related. 

• Place primary y-axis on the left of the axis chart. We read left to right. The most important variable should be on the y axis.

•  When using a bar chart, use consistent, defined color/s.

• Use natural increments when creating line charts (e.g., 2, 4, 6, 8, 10) vs. (3, 6, 9, 12, 16, 20). 

• Plot no more than two lines. Charting too many lines is visually confusing. If there are more than four lines, consider using a panel of line charts and a separate bar chart.

• Use comparable scales when contrasting two or more sets of data. 

• Do not use different line weights to distinguish data sets. The reader may interpret different line weight as ranking. 

• Do not make a bar chart 3D. 3D effects visually skews the data.

• Do not use narrow bars, they cause the eye to focus on the negative space.