manipulating plotted lines on a graph

[buj]

I have plotted lines for three data sets and am required to manipulate the data in a couple of ways in order to make each line start at (0,0).

I have attached an image of my raw graph for reference below.

Essentially I need to extend the linear portion of the line to the x-axis as shown below and then shift the entire line so that it begins at (0,0).

I will repeat this process for each line.