The topk() function in Prometheus and Loki returns the topk per interval. This means that at time T1, the topk series returned is a different set as that on T2. As a result of this, the graph in Grafana may contain more series in total than what you would expect.

As of Grafana 5.3.0 there’s a feature that allows correct graphing of the top N series over a duration using the $__range_s variable, as explained in Graph top N time series in Grafana.

At the moment of writing, this is not supported for queries targeting Loki however (issue #25561).