Using cell concentration
What is a concentration

A concentration of cells is the number of cells per unit of volume.

You are free to choose the units you like

It is very much up to you to chose the units you want as long as they express a concentration. There is no right or wrong. 

You can express your graph in cells per liter or in 100,000 cells per $\mu$l or in billions of cells per ml, all would be correct. Of course, some are more intuitive than others. I would advice you to avoid expressing in cell per square meter or in billions of cells be nanoliter because the numbers will be very small and very large respectively and will not convey much intuition.

A good intuitive unit

I don't know what kind of concentration you have in you data but a priori, I would recommend expressing your concentration in cells/ml is quite intuitive.