Toggle legend entry to add/remove line plot

[rolyp]

We already plan to allow inputs and outputs to be added/removed from the “query context” to allow for varying “scopes” of interaction. For example, one might hide the line chart from our line chart/bar chart example and just see data relevant to the bar chart. Semantically this is equivalent to clamping the demand associated with an output at $\bot$, but we might choose to hide (or grey out) the output. Switching which facets of a facet plot are visible would be similar.

This is probably related to the idea of treating outputs that don’t demand an input, or dually inputs that aren’t demanded by any output, as “inert”. But in this case it is the user electing to treat something as inert.

More generally, it might be useful to be able to “remove” output elements from certain kinds of visualisation. For example in #811, we might remove one of the rolling averages so as not to be distracted by irrelevant related inputs/outputs. Or we might pick a subrange of the x-axis by dragging a slider, with similar effect.

(There is a question of what this would mean if the removed elements were parts of some other aggregation, e.g. a sum or average – would those aggregate plots be updated to reflect a different data selection? If so, we can still understand these interactions as monotonic/join-preserving?)

See also:

• Toggle inputs to change query context #951
• ▽(⊤) as ambient query context #949
• Facet plot #1068
• Aggregate values in BarChart and LineChart #999
• Rolling average example #811
• Legend entries linked to relevant subplots #984

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For example in explorable-viz/fluid-examples#38, @RaoOfPhysics suggested the following:

• All legends (historic data + projection models at bottom of Figure 6.19) should be selectable
  • Additional legend required for emission causes/types
  • Charts should by default show all keys in the legend selected, but user should be able to deselect any keys and have the plots adapt
  • A slider should be built for the date range, allowing you to modify what is shown by moving the two ends of the slider
• Click on a facet (e.g. CH₄ for Africa and the Middle East)
  • All other facets are “deselected” (dimmed) and related sub-plots are shown (i.e. breakdown of emissions by cause/type for CH₄ in AME)
  • Clicking on any other facets should add its sub-plots to the view somewhere
• Click on a row title (e.g. CH₄)
  • Show the emissions for all regions for that particular gas
• Click on a column title (e.g. Africa and the Middle East)
  • Show emissions for all gasses for that region