I need to compute the yield point for an experimental true stress vs true strain plot of a material as attached in the screenshot. See attached; I have true stress vs true strain plot. I took some initial points on the curve and fitted a straight line which gives me the elastic part of the curve. I constructed a line parallel to this straight line (parallel line has same slope as straight line, I got the constant of y = mx + c of the parallel line by putting x =0.002 when y = 0). But from the graph it is evident that yield point is around 1500 MPa but the offset method gives me a yield point around 1235 MPa. Please can anyone suggets/help what is going wrong?