Legend: Only display data points relevant to the visualization

[sosiology]

While testing the improvements to the legends and symbols (issue https://github.com/visualize-admin/visualization-tool/issues/1425), the question arose from Tobi B. if it is possible to only show the relevant items in the legend.

see example here: https://int.visualize.admin.ch/de/v/Ne4JcJneHn8p

[sosiology]

@bprusinowski @ptbrowne @KerstinFaye would be interested in your opinion here. Based on the linked example, it seems reasonable to remove the non relevant legend items but i may be missing something.

[bprusinowski]

I also think it makes sense :)

[Rdataflow]

@sosiology @bprusinowski I agree with one exception: in case of ordinal scales (like forest fire warning) having the full scale helps to better understand the severity of some isolated level

[sosiology]

reviewed on test and LGTM