Render mark

[Fil]

I've made two versions of this (in two consecutive commits):

• 1. the function mark: marks: [ () => svg'<text>function mark' ]
• 2. the render mark: marks: [ { render: () => svg'<text>render mark' } ]

The second solution is imo much better since you can discover it by inspecting a mark. (It might be hard to figure out what initialize does without reading the documentation, but the render function is immediate and simple.)

closes #182

[enjalot]

I agree the second one is more explicit and consistent. someone reading a plot with this kind of mark would also be more likely to deduce what's happening. someone writing a custom plugin would already be likely to understand initialize and its implications.

[tophtucker]

i like it!!

[Fil]

see also #299

[mbostock]

Closing in favor of #498.