finess palettes

[piccolbo]

Right now altair_recipes is using a constant chroma/lightness palette (a hue only palette or HOP), which combines with opacity to show both a variable value as well the number of samples in that area. Some issues:

1. The combination with opacity doesn't work that well, looks pretty dull and doesn't fade into background
2. When the only variable represented is counts, the HOP doesn't blend well with the background. The background color should be close to the color fro count of 1.

Potential solutions

1. We need two palettes, one (the default for instance) that blends into the background and the other that is constant lightness
2. We need to understand batter how a constant lightness palette combines with opacity
3. We need to select one of the two palettes appropriately or punt that to the user. For instance, we could use the standard one when the quantity represented is count(), on the HOP otherwise.