Allow mark channels to specify a default scale type

[mbostock]

Currently mark channels can specify a required scale type, such as barY’s x channel requiring a band scale; but what if mark channels could specify a default scale type that overrides data-based type inference? For example, the line and area marks are typically used with quantitative dimensions (and more specifically, x is often temporal). While it’s possible to use these marks with ordinal data, most of the time it’s a mistake caused by lack of typing in CSV. If these marks could default to quantitative/temporal instead of default to the data’s type, it might avoid a common mistake. Related #532.

[mbostock]

I think this is sufficiently covered by the new warning system, though we could introduce a warning when a line or area is used with ordinal data.