Having a hard time getting a .2% offset to determine yield of material

Hello, I am new to this forum and quite new to Matlab. I am currently trying to analyze data from a MTS machine that took tensile testing data for three samples, an annealed 1018 sample, a heat treated 1018 sample, and an aluminum sample. The data was given as a .txt file which I imported into Excel as I prefer to work with it there. I then imported it into matlab with xlsread and from there I have attempted to create plots for Engineering Stress v Engineering Strain as well as a plot that compares crosshead, extensometer, and extension of the sample. I was able to obtain most of the information I needed such as maximum tensile strength, failure strength of the material, and some other odds and ends using getpts as well as finding the maximum stress using max. However I have not been able to get a correct .2% offset method yield and also I am not sure how to compute the slope of the linear region to get a modulus of elasticity for the three materials. Any input would be greatly appreciated. I have attached two of the problematic files as well as their corresponding Excel files.