Polar 2.0 open items

Followup issue of https://github.com/plotly/plotly.js/pull/2200 and the on-par polar milestone.

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Features we could easily add

• radial and angular axis spikes
• support for radialaxis.side: 'center' to display radial axis labels along invisible radial axis lines
• radial axis hole (similar to pie hole, mpl example) - done in https://github.com/plotly/plotly.js/pull/2977
• support square grid lines for category angular axes example - done in https://github.com/plotly/plotly.js/pull/2739
• some way to snap to NSEW corners during angular drag
• way to activate radial (and possibly angular down the road) autorange on double click (i.e. implement doubleClick: 'autosize' config option
• radial (and angular?) axis zeroline
• polar (r,theta) data-referenced annotations
• trace-specified and radial-axis calendars
• make polar respect gd._context.showAxisDragHandles?

Potential features that require some thought

• How to replace legacy polar’s area trace type? I’d vote for replacing it with polarbar or wedge. Most of the bar logic could be reused here. Moved to -> https://github.com/plotly/plotly.js/issues/2810 Done in -> https://github.com/plotly/plotly.js/pull/2954
• How to replace legacy polar’s (r,t) bar traces? I’d vote simply removing them, unless someone out there finds them useful.
• What to do with r < 0 values? In #2200, they are simply dismissed, but perhaps we should add an option to plot them on the opposite side of the origin. See https://github.com/plotly/plotly.js/pull/2200#commitcomment-26416872
• Date angular axes. See examples in https://github.com/plotly/plotly.js/pull/2200#discussion_r156397924) and some considerations in the last part of https://github.com/plotly/plotly.js/commit/1e81039384ebf89de376c3ddd004d0f103278fa6#r26653662 Moved to --> https://github.com/plotly/plotly.js/issues/4379
• Should we allow angularaxis.period to be less then the number of categories on categorical angular axes? See https://github.com/plotly/plotly.js/pull/2200#discussion_r161143299
• Maybe add r0, dr, theta0 and dtheta. For the theta pair, it might be tricky to find a correct default the everyone likes. Done in -> https://github.com/plotly/plotly.js/pull/2895
• Add another zoombox mode? Perhaps a pixel-based zoom using polar.x, polar.y, polar.zoom scalars to describe it? See https://github.com/plotly/plotly.js/pull/2200#issuecomment-351127938
• scatterpolar (r,theta?) error bars
• Properly clip scatterpolargl line segments

[etpinard]

From @alexcjohnson 's https://github.com/plotly/plotly.js/pull/2739#discussion_r197297290

• we should have a mode in scatterpolar to join the endpoints together so you don’t need to repeat the initial point when the data are meant to be periodic. trace.connectends perhaps, to mirror trace.connectgaps? Moved to -> https://github.com/plotly/plotly.js/issues/3073

[nicolaskruchten]

@elben10 this is basically just a list of ideas at this point, no one is actively working on these that I know of, but we’d appreciate any help anyone can provide 😃