The bin transform doesn’t currently include per-x and per-y BinOptions

[mbostock]

For example, x: {value: "x", domain: [0, 1]}. I expect we need a ChannelValueBinSpec or some such. Ref.

plot/src/transforms/bin.d.ts

Lines 279 to 281 in 29e40b0

	* To pass separate binning options for *x* and *y*, the **x** and **y** input
	* channels can be specified as an object with the options above and a **value**
	* option to specify the input channel values. (🌶 NOT TYPED.)