Support for barcharts

It would be great if the plugin could also support barcharts. Currently, in stock-grafana, bar-chart support - in particular for non-timeseries data such as multi-factorial cross-tabulations - is limited/missing.

Also see https://github.com/grafana/grafana/issues/19994 and https://github.com/gipong/grafana-groupedbarchart-panel/issues/2