Secondary y-axes
You may want to have a chart or graph with more than one y-axis. This can be very useful when you wish to compare trends in two or more data series but where the y-axis scales are very different.
We will use a table of made-up data (Figure 70) showing the number of hikers and the average temperature (in Celsius) in a park over a 6-month period to demonstrate this.
Plotting the data in this table produces a chart that gives a view of the trends in temperatures and the number of hikers per month, but the scale is such that we cannot easily see the changes in temperature since the temperature line is rather crushed down in the bottom of the chart.
We have done some basic formatting, but we did not add a y-axis label because we have two different data series (Temp and Hikers) with two different kinds of scale, Degrees Celsius, and Number of Hikers.
A better approach is to graph the two data series, Temp and Hikers, against two y-axes. To do this:
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Right-click on the chart and select Edit.
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Double-click on the data series you wish to plot against a different y-axis, select Option > Align to second y-axis and click OK.
The result is shown in the chart in Figure 72.
This change in the plotting scale for the Temp gives a much better idea of the relationship between the temperature and the number of hikers in the park. It seems to suggest that most hikers are wimps and don’t hike in cold weather.
