More flexible categorical scales about vega HOT 4 CLOSED

Commit e01e34f adds support for direct addressing of specific color channels in one of RGB, HSL, or LAB color space. Combined with appropriate scales, this may let you do the color encodings you describe in a principled fashion.

Here's an example of setting LAB color values on a per-channel basis. L ranges from 0-100, while A and B channels range from (roughly) -100 to 100.

"update": {
   "fill": {
    "l": {"scale":"y", "field":"data.y", "mult":0.5},
    "a": {"scale":"x", "field":"data.x", "offset":-100},
    "b": {"value": 0}
  }
}

Here's an example for HSL (Hue, Saturation, Lightness). Hue ranges over 0-360 (higher values are taken modulo 360), saturation from 0-1.0 and lightness from 0-1.0.

"update": {
   "fill": {
    "h": {"scale":"y", "field":"data.y"},
    "s": {"scale":"x", "field":"data.x", "mult":0.01},
    "l": {"value": 0.5}
  }
}

RGB works similarly, with each channel ranging from 0-255. Underneath the hood, standard d3 color objects are used for each of these color spaces.

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Awesome. Any chance of a hcl (polar-LUV) space?

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Done: cdd1b98. Just use "h", "c", and "l" as your keys.

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Awesome - thanks!

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