Could we make the free disk space optional on the graph? In my case the free disk space is around 130GB, compared to around 10GB of jobs and builds. So the graph scales to 150GB, and the lines for jobs and builds just sit along the x-axis and don't show any useful movement.

True, I intended more to have a momentary free disk space display as text to easily check this.
The graph would be optional or even not necessary at all.

I quite like the graph - it's just that it becomes virtually useless if the free space changes the y-axis scale too much. I'd suggest taking the free space off the graph - and just report the current free space alongside the other totals:

Jobs:11 GB, Builds:2 GB, Locked builds:-, Workspace:9 GB, Free Space: 224GB

Yep, that's exactly what I also meant

I added the free disk space for jobs (optional in jenkins configuration since 0.23) into graph history, but you are right that current free disk space should be more useful information so I add it too. Maybe it would be nice to have free disk space for slave workspaces too. But it needs to separate workspace paths which is not included into current slave workspace - it could be for example custom workspace path or old slave workspace path which I add into workspace disk usage of job too.