Matthew -

Make a stacked area chart, then unshade the bottom series (Fill = None).
 You plot the difference between the two original curves for the second
series.  If you want better control over the lines, and especially if
you want to use data point markers, make your stacked area chart and
format the border of both area series to None, then add the two original
series, and change their chart type back to Line.

- Jon
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Jon Peltier, Microsoft Excel MVP
http://www.geocities.com/jonpeltier/Excel/index.html
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