Legend on graph changes after adding Transform filter

Grafana Time Series Panel
1

I have been playing with graphing temperatures from an internal sensor. Data source is Prometheus.

When I set up a simple graph I get what I’d expect and the legends show the values I configured for the graph.
image

However, when I try to fitler out 0 values the legend changes to:
image

Am I doing something wrong here or is this expected behaviour?

Thanks

Dave