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

\begin{document}

\LCPPHDiagram[
  fluid=R134a,
  isotherm=true,
  quality step=0.1,
  temperature min=-40,
  temperature max=120,
  temperature step=20,
  labels=true,
  quality label every=2,
  isotherm label every=2,
  quality label pos=0.38,
  isotherm label pos=0.12,
  isotherm style={line width=0.25pt,black!45},
  quality label node style={font=\scriptsize,fill=white,fill opacity=0.75,text opacity=1,inner sep=1pt},
  isotherm label node style={font=\scriptsize,fill=white,fill opacity=0.75,text opacity=1,inner sep=1pt}
]

\end{document}