It’s axiomatic in visual data communication that grid lines help organize graphs in a rational, easy-to-absorb manner. Grid lines facilitate understanding-by-inspection and help you compare key thresholds. Without them, your mind has to draw an imaginary line to make comparisons.
Here is an example of an interface using grid lines to enhance readability:
I write this as someone that has written or project-managed electrical engineering programs with graphing capabilities and graphing features were always a matter of concern. I can, if you need additional convincing, provide references to papers on this topic.
Also, I think this can be implemented through the QVauleAxis class, which I was going to link to but apparently I don’t have the requisite permissions.