Null data causes graph to stop rendering

[Christopher Piggott]

If the query doesn't return data for some mapping it seems to make the plugin just stop wherever it is. It will either not map any more down the remainder of the mappings list or, in extreme cases, won't render. I think there needs to be a null check in here somewhere that says if the column being applied is null write "?" or "null" or something but allow everything else to be rendered and values mapped.

[Jii-Hoo]

You could try using 'OR on() vector(0)' at the end of queries as a workaround. I had similar issues with another plugin when Prometheus occasionally returned null-vector, adding above helped in that case as it returns vector with zeros instead of null.

I was actually seeing similar behavior with Flowcharting, dashboard stopped rendering after opening it and waiting for 5-10 minutes. However, in my case null data vector seems not to be reason, as I have been using above workaround in queries and even when the issue came up Grafana got result vector OK from Prometheus.

I managed to track source for the issue eventually, looks that it was a bit stupidly implemented Grafana variable which loaded lot of metrics at each time range change (i.e. after each data auto-fresh). I changed the variable to be updated on at dashboard load and now everything seems to be working OK (fingers crossed). No idea why it affected to Flowcharting like this.