Extending matplotlib's DateAutoFormatter

I use matplotlib's AutoDateFormatter a lot. But it doesn't quite match my requirements. As well as scalable date formats I'd also like to be able to mark boundary ticks, e.g. first tick of a particular month or first month of a year, using a different format to the rest of the range markers.

Luckily, after a bit of thought, I realised that I could subclass the module, add an extra dictionary of boundary formats, and add some code to use the locator passed to the object by the caller to determine the relative positions of each tick:

( Gory details... )

This method works well when labelling the X-axis but is slightly less successful when it comes to labelling the value of the cursor in interactive sessions. However it has occurred to me that it ought to be possible to fix this by adding soemthing to setFormat() to return yet another alternate format if the locations array is zero length, as is the case when displaying interactive values.