Help - how to only draw negative grid sections

Hi so i’m trying to make a program that displays the output from a data source the thing is that the source will only produce values that are negative on the Z axes, I’m trying to stop it drawing the grid above the x axes.

If you have any ideas id appreciate how to hear them, I’m hesitant to make another ofDrawgrid function with an if statements if there’s a better way.

Thanks in advance