things we want to render once

[Fil]

This example came up in #700 when using a function mark to add a <defs> element to a chart.

When faceting, the defs will be added to each facet, which is redundant (and illegal if they contain ids). There a contrived way of adding them once only, by giving them an internal counter (state), but it would not be a good practice.

In the case of defs that set gradients and textures, we might argue that they are part of scales—unless they are constants… And sometimes what we want to do is to use the first facet to display some kind of annotation that's needed only once.

[Fil]

copying Mike's thoughts:

Some possibilities…

1. I don’t think that marks should be stateful (which would make them harder to compose and reuse in novel contexts).
2. We could have a “defs” property that is parallel to marks where you can put hooks for rendering into a defs element, but that seems very special purpose for this feature, and I expect we want something higher-level for creating patterns and gradients (eventually).
3. We could pass an additional argument to mark.render that indicates faceting. Technically this is already true because the axes argument is undefined when faceting, but we still don’t know which facet is being rendered. I do feel like mark.render already takes too many arguments though.
4. A render hook for scales? That’s really what we want here—the patterns are to support the color scale. If we wanted to support textures as a built-in scale type, we’d need to provide a way for scales to “render” their supporting elements.

[Fil]

A simpler method:

function (_, scales, __, { width, height }) {
  if (!this.done) return this.done = svg`<g><text x=${width / 2} y=${height / 2}>Hello`;
}