Splom open items

Follow-up from https://github.com/plotly/plotly.js/issues/2372, https://github.com/plotly/plotly.js/pull/2505 and https://github.com/plotly/plotly.js/pull/2527

Things that were left out

• Splom traces placed on overlaid axes are broken, see https://github.com/plotly/plotly.js/pull/2505#discussion_r181237788
• Multi-selection across subplot is disallowed. To do so, we would need to share selectedpoints info across multiple splom-generated subplots, see https://github.com/plotly/plotly.js/pull/2505#issuecomment-380855752
• Relayout'ing grid style on hasOnlyLargeSploms: true with scattergl trace(s) fails to redraw scattergl trace(s), see https://github.com/plotly/plotly.js/issues/2562 FIXED in https://github.com/plotly/plotly.js/pull/3067

Things we could do, but would require some thought

• Add sub-trace on the diagonal (e.g. with diagonal.mode: 'scattergl' || 'histogram' || 'box' || 'violin', see https://github.com/plotly/plotly.js/pull/2505#discussion_r177823962
• Add attribute in dimensions that could default the splom generated axes' style (e.g. dimensions[i].showgrid: false would remove grid lines from all axes corresponding to dimensions[i]). Edit: PR https://github.com/plotly/plotly.js/pull/2899 added dimensions[i].axis.type, it would be easy to add more attribute under that container.
• Make one single cartesian <g .draglayer> covering the whole plot area instead of multiple (12 I think) per subplot. This would speed up initial rendering (especially for splom, but also for all graphs with many subplots), make cross-subplot selections a little easier to manage, see https://github.com/plotly/plotly.js/pull/2505#discussion_r181237581
• optimize regl-line2d for grid data
• implement matching axes https://github.com/plotly/plotly.js/issues/1549
• Remove remaining O(n^2) SVG items. In addition to g.draglayer mentioned above, there are two categories I can see:
  • Subplot backgrounds rect.bg - These could be omitted entirely when plot_bgcolor matches paper_bgcolor (and both are fully opaque - which is true by default), otherwise perhaps we could move them into the WebGL canvas though @etpinard points out this may impact pan/zoom performance. UPDATE: optimized when plot and paper bgcolor match in https://github.com/plotly/plotly.js/pull/3057, possible future improvements in https://github.com/plotly/plotly.js/pull/3057#discussion_r222030211
  • Subplot groups .subplot.x<N>y<M> - normally only the ones with tick labels and titles have any visible content in them, but currently they all have some empty groups, an associated clipPath#clip<UID>x<N>y<M>plot, and a few <path d="M0,0"> elements, we should be able to avoid even creating the unused ones.

Related issues that would benefit splom performance:

• faster axis autorange relayout: https://github.com/plotly/plotly.js/issues/2546
• per-axis 'axrange' relayout: https://github.com/plotly/plotly.js/issues/2547
• stash pts selections for pan: https://github.com/plotly/plotly.js/issues/2548
• speed up cartesian axis defaults: https://github.com/plotly/plotly.js/issues/2549
• replacing getBoundingClientRect calls in Axes.doTicks https://github.com/plotly/plotly.js/issues/1988 - done in: https://github.com/plotly/plotly.js/pull/4165

Begin by reading issue #2372 and PRs #2505 and #2527, then separate the unchecked checklist items from those already fixed by later PRs. No source file or test is named; a contributor would need to agree on one remaining item, identify its entry point and tests, and define completion before starting.