fix margins

[Fil]

margins are problematic, they tend to erase smaller contents by giving it negative height or width.

I've fixed the case of "height" in the "horizontal" gridding mode, but it needs to be generalized and unit tested.

[Fil]

Here we have 20 horizontal yearly series of 12 monthly values with valueHeight = max(monthly values) for each year. Adding margins in d3-gridding@0.1.2 will "eat" the content of the smaller series and give them negative heights.

before

When it's fixed we see all the series, and they share the available space evenly as a proportion of their valueHeight:

after

[Fil]

here's an interesting difficulty: if we give to each year a valueHeight = sum(monthly value), then the margins are taken within the height, and the poor month 01 in 2020 is half the size of a similar month in 2019.

[Fil]

more or less fixed in #37