map option for map[XY]

[mbostock]

Fixes #479.

My first take was to support Plot.normalizeX(basis, options) and Plot.normalizeY(basis, options). While that’s more consistent with Plot.mapX(map, options) and Plot.mapY(map, options), it’s not consistent with Plot.windowX(options) and Plot.windowY(options). So, I then decided it would be better to standardize on Plot.mapX(options) and Plot.mapY(options), and only use the two-argument form when you want to declare the output channels explicitly (e.g., Plot.map, Plot.group, and Plot.bin).

For backwards-compatibility we can continue to support the old form of Plot.mapX and Plot.mapY, but I consider it deprecated and removed it from the documentation.

[Fil]

Good! Since we're still in the 0.x, do we have to maintain any sort of backwards-compatibility? And if so, for how many versions?

[mbostock]

We need to maintain backwards-compatibility for Observable until we ship version pinning. Otherwise notebooks will upgrade and break.

Also, I’ve realized that the pattern in this PR is inconsistent with the pattern I proposed for basic transforms in #472, so I want to think about how to resolve this discrepancy before landing.

[mbostock]

Closing in favor of #494.