Fix bad 'g' argument for drawHighlightPointCallback.

See https://groups.google.com/d/topic/dygraphs-users/2KGnXNVNE-c/discussion for context:

> As per http://dygraphs.com/options.html#drawHighlightPointCallback,
> the first argument supplied to the callback function should be: g: the reference graph.
> 
> In the case of drawPointCallback g is indeed supplied.
> 
> But in the case of drawHighlightPointCallback, I am seeing g being undefined.