Hey Bill,
With results as shown on that graph, who cares? Just say, "They're all so close together it doesn't make a lick 'o difference which one you use." As a footnote you might add that Cathar's block kicks all their tails.
Or how about, "Given the skill required for a good paste installation, the differences between blocks is below the threshold of significance."
To your original question, I think the proper answer is "flow" on the X-axis. Whether gph or lph doesn't much matter. The problem with pressure is that no one could or would measure delta-P on a block. What they can measure is overall system flow rate.
Whether you go with flow or pressure, you really need an additional graph that ties the two together. How is anyone to know how much delta-P (or flow) they will get with a given setup? At least with flow they can measure it. If they have your data directly in flow, great. If not, then they would need the second graph to tie flow to delta-P.
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