Legend for `model_id` facet plot

[LucieContamin]

          Found this generally very clear and easy to follow. 

I find the attached plot (faceted by model) a bit confusing, with the legend only showing the one model (the first panel). In the interactive plotly, if you then click to not display the model, only the first panel projection disappears (pictured). I would propose to have the default of this plot to not have a legend, but not sure exactly how to set this up practically (tricky with the option default being true for other plots). Another option might be to allow each of the panels to still be a different colour so the legend corresponds to something meaningful?

Originally posted by @saraloo in https://github.com/Infectious-Disease-Modeling-Hubs/hubVis/issues/2#issuecomment-1749283421

In this case the legend should either be:

• if fill_by = "model_id": each model-id with a different color and with each one a legend item. For plotly, with the possibility to show or not show each one of them
• if fill_by is not set to "model_id": all model-id should have the same color and the legend will refer to the fill_by variable. For plotly, with the possibility to show or not show all of them at once.

[LucieContamin]

fixed with commit #d3e39ea