heatmap is much less dense in color than it used to be

[jowens]

https://gist.github.com/jowens/04e37346513ac9b8570e

For a heatmap, all that extra padding does little good; I think you'd like much more color than white space. Previous VL builds were mostly color, not so much white space. Now it's much more whitespacey.

[kanitw]

@domoritz I think I’ll auto set ROW and COLUMN’s padding to 0 and hide their grid if Y and X are not mapped respectively.