Calculating Elastic Constants of Epoxy Networks

Hello everyone,
I have a question regarding the post prosses calculation of the elastic properties of a bulk polymeric glass system. We know from theory that the stress strain curve of epoxies has a linear part in small strain (up to 5%). I deform my system and I extract the data for the three tensile tests in the x,y,z directions and plot the stress strain curves. However not all plots pass throught the (0,0) point of the plot. My question is when I do the linear fit is it better to fit with:

  1. y = ax+b
  2. y = ax

See the figures below:

This is with the 1 type of fitting

And this is the 2 type of fitting

The first one gives a value of 0.84GPa and the second a value of 0.90GPa.