Read bandSize config if the scale does not exist

[kanitw]

Fix #1244

Now the following no longer throws error

{
  "data": {"url": "data/cars.json"},
  "mark": "point",
  "encoding": {
    "x": {"field": "Horsepower","type": "quantitative"},
    "size": {"field": "Origin","type": "ordinal"}
  }
}

[domoritz]

Can you write a test for pointBandSize?

[kanitw]

Sounds like a task for our junior colleagues :)

[domoritz]

I don't think this is testing the right thing. We should test what happens if there is only x and x and y.

[kanitw]

That's irrelevant for rangeMixins().

rangeMixins() of x and y scales are independent.

[domoritz]

Okay. Maybe I'm missing the point of this fix then. I thought this pr fixes the issue that we don't have a scale when we only define an x scale because a spec only defines y.

[kanitw]

Oh I see. That case is size's scale, not x, y scale, and thus should be in line 422.