\input p-luacoolprop.tex
\pgfplotsset{compat=1.18}
\LCPSetReferenceState{DEF}
\LCPPHDiagram[
  fluid=R134a,
  isotherm=true,
  quality symbol=x,
  temperature symbol={\theta},
  quality step=0.2,
  temperature values={-20,0,20,40,60,80},
  labels=true
]
\bye

.