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

Bắt đầu bằng cách đọc issue #2372 và các PR #2505 và #2527, sau đó tách các mục trong checklist chưa được đánh dấu khỏi những mục đã được sửa trong các PR sau đó. Không có tệp mã nguồn hay bài kiểm thử nào được nêu; người đóng góp sẽ cần thống nhất về một mục còn lại, xác định entry point và các bài kiểm thử của mục đó, đồng thời xác định tiêu chí hoàn thành trước khi bắt đầu.