thank you for your feedback! but I plotted force on the vertical axis, which I got from multiplying the masses by the acceleration of gravity (F=ma). What leads you to think I plotted mass?
I did an experiment where I measured the stretch on a spring from its initial position when masses were added on the end of it, in increments of 50g. I plotted a graph of Force against length stretched and the graph does not go through the origin, or at least it is not linear for forces below...