If you don't show a title or hide the legend, the space reserved for that information is still set aside in the output.
I changed the code so the header will shrink/expand depending on whether this information is going to be displayed or
not.

I added some constants and protected methods to abstract a little bit of pixel counting and to keep it DRY.  It can
easily be changed if it goes too far.

I've never submitted a ruby patch.  If I have done this incorrectly, please let me know and I'll make it right.

thank you for the awesome library!
Adam