Support min: / max: options for y-axis range

[rmosolgo]

I noticed that charts always had 0 as the minimum y-axis value, but I wanted some charts with different ranges. I found a way to add it, what do you think of this change?

TODO:

• this doesn't quite work, the chart is graphed wrong -- as if it's assuming somewhere that the bottom is zero.
  • Ah, found it! It seems like the code supported either a negative min or zero, but it wouldn't use a given min if it was greater than zero. I updated it to use 0 only if no min is given.