Compressive Yield Stress from curve

I have compression test data stress-strain xy for epoxy. How can I determine the yield point? I guess but not strongly convinced that this will be at 0.2% strain. So I'd need to draw a line from 0.2% parallel to the linear elastic region until it crosses the curve. I How can I determine yield using matlab? Thanks.

[dpb]

As always, start w/ plotting the data to see whether it looks at all like you think it should. If it turns out to be a "classic" stress-strain curve you should be able to fit piecewise line segments and find their intersection...but, until you know what it "looks like" you don't/won't know where to start.