@rsp That would be the kreporter googlemaps plugin, not the googlechart plugin.
You might have a look at modules/KReports/Plugins/Visualization/googlemaps/googlemapsviz.js and lookup for the word legend. Should be already implemented in kreporter 4.x
As mentioned earlier we can no longer provide support for KReporter outside SpiceCRM.
@rsp I am running KReporter v3.1 with SuiteCRM 7.13.4 under the same system configuration that you mention. There were quite a few modifications required for PHP 7.4 but nothing really difficult. The hardest thing for me was learning how KReports was written. When you get something that doesn’t work (blank page, errors in browser console, etc.) just examine your nginx log and it will point you to the location of the error.
As far as adding the legend to Google Maps, I see references to a legend in the High Charts code.