Variable section sweep

Let's say you have a box with dimensions sd4 and sd5. 

You need to create datum graphs that start at the heights of those two dimensions and you need to take them down to 0 along the x-axis. 

Then you write a relation tying the part to the graph.  It would look something like this:


where trajpar is you origin trajectory and 200 is the length of your graph. 

The graph should be drawn in the 1st quadrant and there can be no more than 1 y-value per x-value.