I'm in the middle of doing the same thing myself. I broke the problem down into the 3 pieces: the elastic part is an easy linear relationship, and the tensile and compressive plastic deformation zones are curve-fit using "linfit".
I use an "if" statement to put the 3 parts back together, and now have a continuous function from UTS to UCS that is expressed as:
stress(strain)=_*ksi
so I can type:
stress(0.001*in/in)=20*ksi
Generate the chart with a series of strains in the x-axis, and the function solved for the strains in the y-axis.
Have fun!
STF