Clarify format for Plot.plot() style options

[ropeladder]

It's not obvious that the Plot.plot({style: ... }) options such as width or margin must be specified as e.g. "20px" instead of "20" or simply 20. Adding an example and specifying the required format would help clarify.

[mbostock]

Automatically promoting 20 to "20px" is a React-ism; it isn’t something provided by the standard setProperty. But sure, I can clarify that in the docs.

[mbostock]

Also, it looks like Safari and Chrome do support unitless numeric values (quirky lengths) e.g. style: {padding: 20}, but Firefox does not. So support is browser-dependent.