Where Styles Live

The code can manipulate styles in any of three layers:

• In the canvas/axes code, before a mark is created.
• In the mark code, when the mark is instantiated.
• In the HTML code, when the mark is rendered.

We haven't been consistent about where to implement defaults, so here are some guidelines to help cleanup the mess:

• Wherever possible, set default styles - such as the default axis label font size - in the axes / mark instantiation code.
• Where there are context-specific defaults - such as the defaults for a matrix visualization which are different from a generic set of table axes - set them in the canvas/axes code.
• Ideally, the HTML rendering code should never have to supply default styles (there are a handful of exceptions, where the defaults are implicitly affected by other parameters, such as cartesian axis spine position). Rationale: this code may be executed multiple times to create multiple versions of a figure. Also, there could be other backends in the future, and they shouldn't have to replicate more functionality than absolutely necessary.