Keep in mind that a stacked chart depicts a sum of one or more series, not
just each series. Trendlines only work on individual series. You could
either generate trendlines for each series on a non-stacked basis, or you
could add dummy line chart series to the stacked column chart that use
summed data (use worksheet formulas to generate the data), and generate
trendlines on these added series.

Keep in mind also that category values are not necessarily the best
correlating variable to use in a trendline regression.

- Jon
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Jon Peltier, Microsoft Excel MVP
Peltier Technical Services
Tutorials and Custom Solutions
http://PeltierTech.com/
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