Show crosshair over whole canvas, covering the axes.
Instead of the previous commit, I meant to first check in the
original, unedited jquery.flot.crosshair.js, and then my changed
version, but I did wrong and then git reset won't work the way I
expected it. Oh well.
How my jquery.flot.crosshair.js diffs from original.
(Yes, I have submitted a Pull Request...)