Add functions to expose the x and y scales for external use.

[keithrbennett]

I'm very new to this stuff, so please educate me if there's a better way.

That said, I found that I wanted to put labels beside the x and y axes, and needed to find out what the correct x and y positions were for a given row or column. It seemed like the best way was to use the same scale object that the grid object was itself using. So I added functions to expose those scales to the outside. Is this reasonable?