MAVExplorer: Diff expression in saved graph causes scaling issues

**Symptom**
Scaling of the graph is arbitrary and incorrect

**Suspected Cause**
Having an XML file with a diff expression as a saved graph causes issues with axis scaling with other diff expressions

**Replicate:**

1. Start and load file with nothing in .mavproxy/mavgraphs.xml
2. Plot something like diff(HEARTBEAT._timestamp,0) and everything looks good
3. Save graph to save it
4. Click the Test button and see that the axis scaling is wrong
5. Manually enter expression again to see that manual still has proper scaling
6. Reload saved graphs from menu to load in the new preset graph.
7. Manually enter expression again and now it also has bad scaling

Start MAVExplorer and reproduce the sequence using an empty .mavproxy/mavgraphs.xml, the diff(HEARTBEAT._timestamp,0) expression, and the Test button. Then trace saved-graph loading and axis scaling for diff expressions; done means saved and manually entered graphs retain correct scaling after saving and reloading.

Written by the indexing model from the issue text.