Have you cloned emoncms and the graph module at the same time or updated them together (git pulls)?
Are they both master branches?
There was a change a couple of weeks ago to the graph module that depended on a change to the emoncms core too. So graph modules installed or updated after that point need to run with emoncms installed or updated after that time too, assuming they are both on the default master branches.
If they are both master and both up to date, you should be ok
If they are both stable branches, you should be good.
It’s only when you have latest graph master and out of date emoncms master or emoncms stable, you should see the problem.
In short, ensure they are both master or both stable and ensure they are both up to date (fresh pull).
If the issue persists after an update and a browser cache clear [CRTL-F5] let us know the output of the following command, run from each of emoncms and emoncms/Modules/graph folders
[edit - split 2 commands on to 2 lines in case the first fails (see below)]