How can I correctly subclass AutoDateFormatter and use it in my code? 

What I am doing is copying the code from matplotlib's AutoDateFormatter and changing the strings for how the dates are represented and making that a class, MyAutoDateFormatter.  AutoDateFormatter expects a locator, and I think (?) the default is AutoDateLocator.  So in my code I am doing this:

        adl = AutoDateLocator()
        myformatter = MyAutoDateFormatter(adl)
        axis.xaxis.set_major_formatter(myformatter)

But when I run it, no matter the level of zoom, it says "2010" (when it should change depending on zoom level). 

However, if I go into the matplotlib dates.py code itself and save the same changes to the date strings there, and I comment out the above code, then it works:  the date strings change depending on level of zoom. 

I could just leave it changed in mpl's dates.py module, but I'd rather subclass AutoDateFormatter so that if I share the source code of my app with others, it will get this right.

What am I doing wrong? 

Thanks, and any help appreciated as always.

Che