Support using a log scale for heatmaps

[Dan LaManna]

It would be useful to support using a log scale for generating heatmaps with wide distributions.

Edit: by the way, thanks a ton for this tool. I've avoided learning gnuplot for years and rare looks like it will help me do that indefinitely 😄.

[Chris LaPointe]

Thanks for the issue, that's a good idea, and should probably be applied to more than just heatmaps. I'll try to take a pass at it in a few weeks.