Custom styles for axis and ticks.

[mbostock]

What if we want to change the color and font of axes?

[morozovkirill]

It would be nice to set different font size for ticks on different axes. Now it's impossible even through class name selector. May be name the styles like "tickX"/"tickY"?

[Fil]

Related: #253 ?

[irenedelatorre]

It would be nice to add a class in the axis group (axis-x, axis-y). This is something that d3.js does, and it lets you style the axis as the user wants / needs. Plot already includes this information in the aria label.