Meta-issue: All the things we punted because of VegaLite

[thomasp85]

This meta issue exist to enumerate all the omissions we've made due to VL not supporting it that we can now begin to add (often cheaply) because we have Hephaestus.

• Arrow ends
• Free space on facets

[teunbrand]

I'd like to generalise the 'free space on facets' to any panel size tooling, also panel.widths, panel.heights, aspect.ratio equivalents.

• Theming (Theme system #334)
• Plot composition (Plot composition #531)
• Tables
• Rich text
• Formula display
• Text boxes à la geom_label()
• Control over text lineheight
• Continuous interpretation of hjust/vjust in text (VL has 3 discrete options for each direction)
• Maybe hjust = "outwards" and vjust = "inwards" based on relative position in panel.
• Curved lines à la geom_curve() or bezier curves
• Line ends / line joins
• Raster layer à la geom_raster()
• A common use case of annotation_custom() is inserting images into plots, also in context of branding plots.
  • Vegalite has an image mark, but we've not implemented anything along these lines (Add image layer #522)
• Fill gradients
• Fill patterns
• Tag labels
• Alt text options
• Axis ticks customisation (major, minor, log)
• Strip backgrounds (though arguably can be handled by rich text with text box support)
• Distinction between major/minor grid lines
• Secondary / duplicated axes
• Custom legend glyphs