add: support for natural log scaling of the X & Y axes - #302

The first commit only added log scaling to the box graph; this was a miss. The second adds it to line and histogram as well.

Thanks! It's great!

@shenwei356 I suspect there's a way to tell plot to write labels for the bottom part of the axis -- note that it starts at the same value that it does unscaled, which means there are no ticks for over half the axis. I am unfamiliar with the plot package, though, and I'll have to look at it more to see if that's possible.

If you figure it out first, great! Otherwise, sooner or (probably) later I'll submit a PR for getting it to draw the early ticks on the scaled axis.

Thanks for the quick merge.

Edit:

oh, duh. There it is, right at the top: LogTicks. I'll mess with this and send a PR for it.