A larger default marginLeft for y axes

[mbostock]

The current default of 40px is often too small, requiring a custom marginLeft. Since we don’t automatically increase the left margin based on the tick labels #383, we should increase the default, say to 60px. And likewise marginRight if the y axis uses the right anchor.

We should also consider an even larger default if the y scale is ordinal, since ordinal names tend to be longer than numbers. Perhaps 80px. The same applies to the fy axis.

[mbostock]

Another consideration is that ordinal scales tend to use the center labelAnchor, which consumes additional margins. So perhaps the default margin could adjust based on whether there is a centered label, too.