Is there a reliable online reference for head losses of PEX tubing at various nominal sizes and flow rates?
This article ("Minitube Distribution Systems for Hydronic Radiant Floor Heating", by John Siegehthaler) has a somewhat small / low-quality chart which doesn't seem to cite a refererence / source:
Maybe this is useful at least to get you "in the ballpark".
Uponor "Pressure Loss Tables" could be useful; example:
Although not a graph I believe you could generate a curve from this data; and it it is usefully broken down by tube size, fluid type, and temperature.