MCT-3898: [Plot] Improve DN/EU and ERT/SCET/SCLK labels.

- Display plot labels for selected Non-time values. 
Displays on top of the Plot view graph. e.g. (ERT, DN)
- Also is shown in the mouse hover coordinate points. e.g. (ERT, DN)     (X: 224/23:00:00 ERT, Y: 0.500 DN)

Wrote or re-wrote unit tests (Y/N): N
Ran entire suite of unit tests successfully (Y/N): Y
Verified bug fix with the described replication steps or verified the implementation meets the requirement cited (Y/N): Y
Code coverage requirements satisfied (new code=80%, changed code=at least as high as existing) (Y/N): N
Code review date and attendees: Code review by pull request
Describe potential impact on other areas of the code: Plotting
Describe any test prerequisites for this fix (e.g., restart the database): N
For bugs marked "Regression - Yes", describe the cause of the regression:
Did you modify any pom.xml file? If yes, did you thoroughly document your changes and reasons? N