Added block chart

Hey Terry,

I don't know whether you accept new graph types. However, I already made a 'block chart' based on your (bar chart) code. It uses the same structure so I think it'd be a nice addition to the project.

You can find the class and a demo in this PR. The readme and changelog aren't yet updates for this addition. I could look into that if you would accept this addition.

Cheers,
Jonathan