Up to now I've been sampling data about total disk space one time each day however issue is that this is causing not drawing line with total disk space in case of observing graph in less than 24h time frame. I found that now is possible to apply preporocessing filter "Discard unchanged" and "Discard unchanged with heartbeat" and I've started sampling those data every 20min. It caused of course that after sampling first point in that metric to the history table no longer are added new values.

That is really useful feature because it allows reduce rate of the data written into the database. However I found that after enable that filter graphs are not as I would expect.

Graph is drawn out of only raw data stored in DB despite fact that in that case values of the metrics after last stored in DB backed are well know. 

**Steps to reproduce:

1. Define item which value will be not changing like "vfs.fs.size[/,total]"
2. Define graph which will be drawing graph of that data
3. After sampling initial vale there is no drawn line beyond sampled vales

Result:

Result is as it is in the attachments.

I would expect that if that values (because metric is with throttling filter) after last stored values are well known that graph will be drawn without gaps after that that point in metric data using last value populated to the end of the graph.