ANSWER BIG QUESTIONS

[paleolimbot]

• Where is the line between a ColumnMapping and a Layer? Should the data be part of the ColumnMapping? Should the data field of either update throughout the build process?
• The line between a Coord, a Facet, a Builder, a Graphic, and the Renderer are poorly defined. As much as I was trying to avoid it, the Coord and the Facet probably should have a $render() method that do the equivalent of Renderer$render_panel() and Renderer$render_panels(), respectively. This means renderers need their own Coord, Facet, and (probably) Builder implementations.
• Should a common nomenclature be used for size, colour, fill, etc.? Or should renderer-specific values (e.g., pch, lty, col) be used?
• The Renderer should probably have just one render function, render_geom(geom), but this won't be possible until the geom package actually exists (and works)
• In addition to choosing the position aesthetics, the Coord should also assign default scales/train them (keeps geometry and x/y/z aesthetics in sync)
• Should the concept of a frame be built-in? This might allow native animation (for static output types), or at the very least, a huge number of facets split into multiple plots (like ggforce::facet_wrap_paged())
• Are they the Layer's Panels, or the Panel's Layers?