Plot: How to adjust graduations (or tick marks) of y-axis ?

-125 change to -120.

You can edit the bottom label of the Y-axis, and the one just above that and the top label. With these three you should be able to find a neat distribution of labels along the y-axis that includes 0.
e.g. respectively -140, -120 and 20 should do the trick.

Success!
Luc