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?