never round scales that would result in 0 bandwidth

[Fil]

fixes #72

[mbostock]

There’s a related problem with bars/rects with rounding off if you set insetLeft or insetTop, as the bin mark does by default. Also, having a zero bandwidth is fine for certain mark types, such as rules and dots. What if the bar mark had a minimum width in its ordinal dimension, say, 1px? And further, what if insets weren’t allowed to reduce the corresponding dimension beyond 1px (e.g., for a rect that’s naturally 1.5px wide with an insetLeft of 1px, the insetLeft is implicitly reduced to 0.5px).

[Fil]

I wonder if this is not an issue to address in d3-scale:

- if (round) step = Math.floor(step);
+ if (round) step = Math.floor(step) || Math.sign(step);

https://github.com/d3/d3-scale/blob/master/src/band.js#L26

[Fil]

We should do this in d3-scale d3/d3-scale#243