If so, how do I correct it and make it more diagonal? Two cell lines (cancer cells) show FSC-A and H like this. I tried to play around with voltages but could not get a more "diagonal" scatter plot for area vs. height. Any help is appreciated.
the data looks fine for me. It seems you do not have doublets. Just mention that cell line 2 seems to have a more heterogeneous cell population in terms of size: higher FSC-A value, so bigger cells.
In any case, if you want to see the data more "diagonal", you can just reduce the Y-axis range; i.e. make FSC-H axis to measure until 150K a.u. maximum. That way, it will appear as diagonal.
Maybe that way you can separate some doublets (some points that have FSC-A > FSC-H, but I think it´s very subtle).