Steps to reproduce:

1. Send values to item
2. Create graph widget, add legend with displayed values
3. Open widget configuration form, update widget, open form, close the form etc. (see video) and observe, how the values in legend change.

Note: Values used while testing:

Result:
See video: graph widget values.mp4 
Expected:
I suppose the expected result is that values are not changing when edit form is opened or closed.