ENH: Add option to highlight NaN cells

[sburns]

Useful for quickly inspecting data in an ipynb

• Add display.highlight_nan option (default True)
• Add argument to DataFrame.to_html
  and HTMLFormatter.init
• Add test

[ghost]

Thanks for the PR, but this solution is too specific. The actual feature needed is
conditional formatting and it needs to be solved in a more general way, one
which will support this as a private case.

closing.

related #3190

[hmeine]

This is nearly exactly what I have in mind. (I would like to de-emphasize NaN cells.)

My idea was to add a class to empty cells:

<td class="pandas-nan">

Then, one can use CSS for arbitrary styling. (No, even CSS3 does not have selectors yet based on content which would allow to select NaN-cells.)

Since CSS allows many other types of conditional formatting already, I would say that this is a useful addition that does not interfere with other enhancement ideas (templates, ...)?!

[sburns]

I like you're solution much better @hmeine but it's for @y-p to decide.

[cpcloud]

@hmeine @sburns We might be waiting quite a while for @y-p to decide since he's stepped out of pandas dev. I think may be too specific. We don't really want to add anymore arguments to to_html, though I understand the need to be able to customize HTML output. If one of you wants to take the lead on #3190, that would be most welcome.