Single-toggle of grouped traces in legend

This is a feature request.

Please see default “Grouped Legend” example: https://codepen.io/plotly/pen/qjmNXa

Current behavior

It is possible to toggle single traces by clicking on them in the legend. If traces are grouped using legendgroup (like in the example above) not the single traces but the whole group is toggled.

Future behavior

It should be possible to disable the toggling of the whole group and instead toggle the single traces like in no-grouped legends. This would make it possible in the example above to only disable the “points” and only show the “avarage line” of this group.

This should be done by a new option in the legend-settings.

Related: #65, #665, #674

[rithwikjc]

I would summarize the suggestions as follows.

• It would help to have a legend group title for each legend group. Clicking on it can act as toggle for all scatters of that legend group. This also takes care of #689.
• Each individual scatter is grouped under it’s respective legend group title, and the toggle for each scatter works as it usually does for the individual scatters.
• Legend groups can be toggled by something like legend_legendgrouptitles in layout (this is global, meaning all legend group names are displayed or hidden for the particular plot), and each scatter is handled already as done by showlegend parameter of the scatter.

This solution should take care of most use cases and issues relating to legend group title and (scatter/group) toggles, except for the very particular use cases like selectively displaying legend group titles for certain legend groups.

The legend would look like this after such a change:

[mcstarioni]

I would suggest a following behaviour: legend group would become an independent toggle with a name of a legendgroup, which would activate/deactivate all contained traces. Yet if someone needs to toggle some particular trace, he can add showlegend=True, and displayed legend for this trace would toggle only this trace.