Yeah, I see what you're saying, it doesnt make sense to use 0.2cm onward. I did try to use stress/strain values at 0.1cm intervals but after coming up with the answers, I somehow convinced myself it wasn't right and decided to use 0.2 instead. Thanks for pointing that out.
I've redone the...
'Gummy 1' = Black Line
'Gummy 2' = Orange Line
Approach:
Using the diameter given and the force measurements on the vertical axis of the graph, I've worked out the cross-sectional area of the mold to be 0.000706858, which I've rounded to 0.00071.
Using this data and the equation Stress = F/A...