In the XL world a trendline only applies to one series. You can create
multiple trendlines, one per series. Alternatively, plot all the data
as a single series and create a trendline for this composite series.
If you want the chart looking like it does now, add the composite
series to the chart and format it to be invisible.

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In article ,
says...
I have one graph (x,y scatter) with 8 series (plots) on it. I want to assign
a trendline but it only gives a trendline for that series. Is there a way of
assigning a trendline to give a best fit for all the points in all 8 series?

any feedback would be much appreciated

A. Gray