Add opacity to aesthetic channels.

[bmschmidt]

Currently opacity can only be set globally for all points, but there are cases where you want to have a subset of points at full opacity, e.g. #35. This is pretty straightforward to implement.

[davidnmora]

I second this, this would be really nice!

[davidnmora]

Just want to spec this out a bit. There's a lot of ways it could go:

@bmschmidt is this what you had in mind, supporting both options a & b?

  {
    encoding: {
      opacity: {
        // Option a: just map the domain of a field to opacity [0, 1]
        field: "percentage_of_some_thing",
        domain: [0, 100],

        // Option b: a lambda that returns the opacity value
        lambda: some_field => some_field !== "jazz" ? 0.3 : 1,
      },
    },
  }

[davidnmora]

Looks like I'll dive into this by creating a rough "clone" of the Size aesthetic (similar API), and then from there we can tweak the API & options to get it just right!

[davidnmora]

For reference form our convo on how categorical columns get handled.