Avoid drawing twice

[andrewdmay]

The current plugin behavior that causes draw to be called twice is documented in the source code as a hack due to flot not having a hook in a suitable place.

It seems like that should be listed as an issue with the plugin even if would require changes to flot to fix.

Additionally it would be useful to document the current behavior and the performance implications it has as I only discovered what was happening when debugging a callback that was being called twice and discovered the comment in the plugin code.

[markrcote]

I posted a question to the flot list to see if we're any closer to being able to drop this hack: https://groups.google.com/forum/#!topic/flot-graphs/Ujp0LAFocqQ