Fix `plot_phi` function
Created by: wilsonmr
Before implementphi into master I stupidly changed the datalabel in plot_phi
to be pdf.name
as in plot_phi_pdfs
even though pdf is never defined or imported in the plot_phi
function and so that function doesn't work.
I have fixed this by reverting the datalabel in plot_phi
to something more generic which I think is fine because there is no legend on the plot.
This was overlooked because I was focusing on plot_phi_pdfs
at the point of merging