\documentclass[border=3mm]{standalone}
\usepackage{luacoolprop}
\pgfplotsset{compat=1.18}

\begin{document}
\LCPSetReferenceState{DEF}
% The diagram identifier is separate from the options; PH, PV, TS, HS, and
% registered custom implementations use this generic command shape.
\LCPDiagram{PH}[
  fluid=R134a,
  isotherm=true,
  quality step=0.25,
  temperature values={-20,0,20,40,60},
  labels=true
]
\end{document}