Custom properties for data source instance ?

No, you are correct -- datasources store time series numerical data only. Various datasources tie themselves into knots trying to workaround this limitation via datapoint legends.  I recommended a while back adding a per-datapoint enum facility so those could be properly displayed in charts as meaningful strings, especially since legends sometimes get so long and don't wrap that you literally have to open the DS source code to find out what a value means. I never saw even a peep from LM on that sensible fix, sadly.