Plot boundaries

[christopher-wolff-zz]

I would like to add a thin boundary line around a plot, just like the axis lines, but can't find a way to do this with the current API. I believe this would be a useful feature, especially for rectangle plots like this one: https://puu.sh/CRRuS/22c509210f.png. Otherwise there are some asymmetries in cells on the right and top boundaries.

[adidahiya]

Have you tried doing this with CSS? I was able to do this in the heatmap example:

[christopher-wolff-zz]

Thanks for the solution!