Comment 12 for bug 1674143

I found that we still have to call `x2p` and `y2p` to figure out which pixels the coordinates are on. Anyways, I instead decided to disable the `DrawLine` calls inside that loop and instead I have them use the cache which I generate. Because the cache is an std::set, all the draw calls now will be unique. This has sped up the first graph draw considerably for me.

Can you take a look at this patch Maciej? Hopefully you can experience the same perf boost it's giving me.