When the range of values for different data series in a 2-D chart varies widely, or when you have mixed types of data (such as price and volume), you can plot one or more data series on a secondary value (y) axis. The scale of the secondary axis reflects the values for the associated series.
Click the data series you want to plot along a secondary value axis.
On the Format menu, click Selected Data Series, and then click the Axis tab.
Click Secondary axis.