def plot_hits_per_event(tree, label, printer):
"""
Call hist_hits_per_event and make plots from the results.
<label> is a short filesystem-compatible string describing the tree.
<printer> is some callable taking one argument, <name> which is a
short, filesystem-compatible string describing the currently drawn
canvas. The canvas must be available as a <.canvas> attribute.
Return a flat list of filenames printed to file.
"""
printed = []
h_ds, h_us, h_dsus = hist_hits_per_event(tree, label)
h_ds.SetLineColor(2)
h_us.SetLineColor(4)
printer.canvas.SetLogy(True)
h_ds.Draw()
printed += printer("downstream")
h_us.Draw()
printed += printer("upstream")
ustat = move_stats(h_us)
ustat.SetLineColor(4)
dstat = move_stats(h_ds, y=-0.25)
dstat.SetLineColor(2)
h_ds.Draw()
h_us.Draw("same")
ustat.Draw("same")
dstat.Draw("same")
printed += printer("dsandus")
printer.canvas.SetLogy(False)
h_dsus.Draw("COLZ")
printed += printer("dsvus")
return printed
