Make map scale nice and rounded

[noahmanger]

Can we round the max on the map up to the nearest power of 10 (or something)? Right now, the scale gets a bunch of seemingly awkward numbers because the max isn't rounded.

So in this case, maybe the actual max of 184.2k is rounded up to 200k with intervals at 40k, 80k, 120k, 160k, 200k. Make sense?

[noahmanger]

Possibly useful for legend-making: http://d3-legend.susielu.com/

[noahmanger]

Also this is another idea @shawnbot shared that could be useful for the scale: http://bl.ocks.org/tmcw/4969184

[shawnbot]

If you're using a linear scale you can also use scale.nice() to round the max up to a "nice" value.

[leahbannon]

Addressed in #445