redshift in astrophysical plots
Redshifts are plotted because usually that is all that can be measured.
Other quantities could be plotted, but since these are inferred from the redshift itself (e.g. density would just mean plotting $(1+z)^3$), or would need to assume particular cosmological models or parameters (e.g. if you plotted look-back time), then it makes sense to stick with plotting the observed data.