A secondary value axis makes it possible to compare data series that fall within divergent ranges. The secondary axis, positioned at the right, can have a completely different scale from the primary axis. You can assign as many series as you like to the secondary axis.

Figure 21-15 illustrates the use of a secondary axis. The series showing cost per square foot occupies a range so much smaller than that showing median square footage that it’s difficult to know how or whether the two series relate to one another. Putting either series on a secondary axis solves the problem.

Figure 21-15. Moving either of these series to a secondary axis makes it easier to see whether and how the two sets of numbers might relate to one another.

To move a series to a secondary axis, right-click it, click Format Data Series, select the Series Options category, and then select Secondary Axis.

As mentioned in Chapter 19, you can distinguish one or more data series from the rest of a chart by assigning it or them to a contrasting chart type. To do this, right-click a series, and click Change Series Chart Type.