Graphs need to support clipping of lines to border of graph

[kms15]

If a path contains a large finite value - say (1, 1e99) - the canvas widget in Firefox will refuse to draw the entire path (including lines that are within the window). Currently the graphing code tries to draw lines this way, and this makes it very difficult to diagnose numerical stability errors, because you can't see the behavior before the equation blows up.

The correct way to fix this is to manually clip the line segment to the four edges of the window; we can't just clip the x or y value. It is possible to have points with extremely large but meaningful screen coordinates (e.g. when manual zoom is added), and a line from 0 to 1e11, 2e11 should still have the correct slope.