Print group name and number of items in bin.

def clicked(self, event):
        """Print group name and number of items in bin."""
        group = event.artist._mt_group
        n = event.artist._mt_n
        dt = num2date(event.artist._mt_bin)
        print("%4i %s events in %s sec beginning at %s"
              % (n, group, self.bucketsize, dt.strftime("%b %d %H:%M:%S")))