Plot object access

There are times when developers might need direct access to the Flot `Plot` object.  For me, it was to be able to call the `highlight` and `unhighlight` methods.  These are (presumably) specified outside the `dataset` (by Flot) because they involve the overlay layer.  So it isn't possible to use just the `dataset` variable to specify how highlight should be done.

I thought about simply adding some data to specify highlight information in addition to `dataset`, but I was concerned about not having a rich enough "API" to handle all cases.  This approach leaves it up to the developer to decide what to do.

I should point out that this is 100% backward compatible and doesn't make things any more complicated for the 95% of cases that don't involve needing the `Plot` object.  But for those 5% cases, it could be useful.

Let me know what you think.