Sankey: render a thin but visible node even if value is very low

[monfera]

Fixes #2015 (see before/after example there)
Issue found when looking at #2014

Jasmine test produces this DOM tree:

[alexcjohnson]

Good catch, and really nice test!
💃

[alexcjohnson]

though - just a nitpick, but it doesn't look like the node is aligned quite properly with the link in this case?

[monfera]

I can put a bit more into alignment but maybe it could be an independent ticket:

The alignments, in general, aren't perfect. When developing the sankey, there were tradeoffs due to these factors:

• crisp, uniform thickness node borders requires that SVG shape-rendering is set to crispEdges - compare Mike's original with our take which gives more consistent apparent box borders
• even if we have this, constant node border (stroke) thickness and constant color requires that the rect borders are snapped - for example, moving a line half a pixel can result in a less contrasty, blurred line (it's done with things like adding 0.5px and ceil/truncate)
• there were some cross-browser inconsistencies (can't remember)
• d3-sankey is a black box and returns node geometry and link geometry separately (also, it has no awareness of pixels or snapping)
• unlike d3-sankey, we more clearly support multiedges - for this reason, we fill shade rather than stroke shade the links - and various options lead to various overlaps between adjacent multiedges, so if we further tweak alignment, we also need to ensure there's no negative banding artifact with multiedges

[alexcjohnson]

@monfera I see what you're talking about with regular (non-tiny) nodes, and I can see why you did it the way you did. I think the normal case is fine, strikes a good balance between the competing visual requirements. But the new tiny one really seems to be hanging off the bottom of the link. I think it's worth spending a few minutes fixing it just in this special case.

[monfera]

@alexcjohnson ah got it, will do!

[alexcjohnson]

#2021 addresses my comment, so this should be good to go! 💃