Ability to invert y-axis (e.g. on line charts)

[davidtcook]

I'd like to have a line chart with 1 being the top value on the y-axis, values increasing downwards.
In my case, this is to chart rankings in a sporting club over time, I'm sure there are other uses as well.

[donv]

Should be doable. I am working on other projects at the moment, but I can merge and release a pull request if submitted.