Problem description: It is not possible to set override Missing data to "Last known" in Overrides tab in Graph widget configuration form: the following error is returned:

Invalid parameter "Overrides/1/missingdatafunc": value must be one of 0, 1, 2.

Steps to reproduce:

1. Open any existing Graph widget configuration
2. Switch to Overrides tab
3. Set any of the configured metrics in Overrides field
4. Add the following override to it: Missing data = Last known
5. Attempt to save widget configuration.

Result: widget configuration is not saved. The above error is observed.
Expected: widget configuration should be successfully saved.