Add ability to disable legend trace toggling

Some folks don’t want it apparently; we should make this optional.

Maybe a boolean layout.legend.tracetoggle attribute?

[tudor-sv]

I also had this need to disable trace toggling, but only for a particular trace, not for all. I wanted to have like a “background trace” that would be displayed in the legend but be untogglable. After digging around for hours in the plugin’s js , I made an accidental discovery while inspecting the chart’s svg dom elements: There is a special transparent element overlaying each trace in the legend (.legendtoggle). This element has the attached js click handler responsible for toggling the corresponding trace. By making this element unclickable, we circumvent the need for any new js hacks or changing the plugin api. Here is the workaround, using only a few lines of css: http://codepen.io/tudor-sv/pen/vXEZYG Of course this method could also be used to make all legend items untogglable…

[jacobq]

This seems to be possible now with the plotly_legendclick & plotly_legenddoubleclick event hooks by returning false from the custom event handler. See https://stackoverflow.com/questions/51029619/what-is-the-right-way-to-override-default-event-handler-behaviors-e-g-toggle-t