Make the chart (I assume it is XY) from two separate ranges.
Start with a chart made from weeks 1 thru 5 data.
Now select data (both the x and y columns) for the weeks 5-20; copy; click
chart; use menu item Edit | Paste Special and specify New Series with
x-values ion first column
Now you can add trendlines for the separate series
You can format the series to look identical if that is your need.
best wishes
--
Bernard V Liengme
www.stfx.ca/people/bliengme
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"dshearer" wrote in message
...
I am trying to add two different trend lines to the same set of data on a
line graph but cannot do it!

To give you more of an idea:

I have been running a psychological experiment where i have been observing
the occurence of a specific behaviour each week for 20 weeks. However, at
week 5 i introduced an intervention which should change the measured
behaviour. To demonstrate this, i need to display a trendline for weeks
1 -
5 and a second trendline for weeks 5-20.

Does anyone have any ideas how to do this?