The dual Y-axis charts raise many eyebrows in the data visualization circles. They are often considered to confuse and lead to wrong data interpretation. However, when you have limited real estate and you want to quickly establish the relationship between 2 variables, the dual Y-axis chart can come in quite handy. Using a dual Y-axis chart, you can easily validate/invalidate relations between two variables with different magnitudes and scales of measurement, as well as gauge a general idea of the trend. However, use it with discretion.
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Here are four key tips for using the dual Y-axis chart: 1. Use the Y-axis on the left for the primary variable and the one on the right for the secondary variable Our brains are conditioned to look for the Y-axis on the left of a chart. To take advantage of this, use the Y-axis on the left for the more important variable. On a Sales Vs Profits chart, when you want the focus to be on sales, use the primary Y-axis (on the left) for sales. Conversely, if you want the primary focus to be on profits, put the profits on the primary Y-axis. 2. Color code your axes and data labels Color coding your axes names, data labels and the data plot helps the user identify the axis with its corresponding data plot. It visually brings out the difference in scales of the two axes. Your user will ultimately save a lot of time that would otherwise go to waste looking up and down the chart. 3. When the dual Y-axis is used to plot the same variable in different units of measurement, synchronize the Y-axes scales. When we use the dual Y-axis to plot the same variable in different units of measurement say Centigrade and Fahrenheit or Pounds and Kilogram, it is best to synchronize the two Y axes. The single data plot can then stand for both the units of measurement and users can easily find the value at a specific point, simultaneously in both the units. 4. Avoid using the same chart type for both data sets Generally, columns are good for discrete categorical data that is measured at standard intervals and used to facilitate precise comparisons. Line charts, on the other hand, are good for discrete data that is continuous and is used to facilitate an understanding of the overall trend/transition. Our choice of charts should usually be determined by the kind of data analysis we seek. But in a dual Y-axis chart, when you have to facilitate transition or comparison for both the variables, the same chart type for both data sets can interfere with each other. Lines may “meet,” which – if drawn on the same scale – would not be anywhere close to each other. Even Columns may look quite close, when in reality that may not be the case. While this problem with the dual Y-axis chart cannot be totally avoided, it can be reduced if we use different chart types for the two variables. Though the Line and the Column may still “meet” but their different forms make them visually distinct from one another. A quick word of advice: Use the dual Y-axis only to understand the trend of the data and not to measure the value of change. Shilpi Choudhury is a writer, blogger, design enthusiast and an amateur actor. She currently writes for FusionCharts. You can get in touch with her on LinkedIn. Udhaya Kumar Padmanabhan heads the UX and Communications initiatives at FusionCharts. He is an invited member of The Society of Industry Leaders (SIL), founding member of ACMSIGRAPH Bangalore and Local Ambassador of UXNet, a global network of UX pros. You can get in touch with him on LinkedIn. All charts have been created using FusionCharts Suite XT. References: Dual-Scaled Axes in Graphs, Are They Ever the Best Solution? (pdf) – Stephen Few Creating More Effective Graphs – Naomi Robbins The Wall Street Journal Guide to Information Graphics – Dona Wong Making Data Meaningful Part 2 (pdf) – United Nations Economic Commission For Europe