Dear @wtf,
coming from Hello from a group of Italian citizens interested in environment and environmental measurements and Access to InfluxDB or Grafana instance with data from luftdaten.info for Italy, we would like to know if you see a simple way or plugin for Grafana to count the “over the threshold” values within a given time-range?
Thanks already!
Friends of precise wording will say at this point that you must be talking about your database and not about Grafana. 
So assuming we still talking about InfluxDB: You would like to filter for field-values above a certain threshold, e.g. with P2 > 50 in the beginng of the query and generate daily mean within the timefilter, e.g. with time(1d). But to my understanding, this would most-likely end up with wrong results cause I think the filter is applied on the raw data first and not the aggregated daily values.
So the better approach would be to build a little more complex flux-query for this. There you can apply filter and aggregations in the order you like. I haven’t done such a query yet, but I feel that I will probably do so very soon and post about this here again.