Hi
From Excel help
When the range of values for different data series 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.To plot a data series along a secondary axis, click the series, and then click Data Series on the Format menu. Click the Axis tab, and then click Secondary axis.
(From Excel97)
HTH
MTB