I ran into this topic when searching how to completely show a graph for the complete selected time range of the graph. Last week I first started with Grafana and was positively surprised by how easy I was able to create a Dashboard!
I have the same situation as described by @zamb: last known data is outside the current scope/zoom (= selected time range of the graph)
So lets say I have data points for a specific measurement each 3 hours: 9:00 / 12:00 / 15:00.
I use in the query fill(previous) (or fill(lineair)) to create lines between the data points.
Result for the graph is that it is perfect when the time range is large enough, so for example 2 days. However when I zoom in to some hours the line is not plotted over the complete time range. The data is available, however outside the shown time range.
So if the time range of the graph is 11:00 - 13:00 it will only plot the line from 12:00 onwards. Is there a way to let the grap also draw a line between the 9:00 and 12:00 data point in this case?