\documentclass[border=3mm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\usepgfplotslibrary{autonode}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
  width=9cm,
  height=6cm,
  grid=both,
  auto node placement,
  auto node debug=summary,
  auto node algorithm=repair,
  auto node failure mode=allow-minimal-overlap
]
  \addplot[blue,thick,domain=0:4,samples=40] {x}
    \pgfplotsautonode[preferred pos=0.65,sloped=true]{$y=x$};
  \addplot[red,thick,domain=0:4,samples=40] {4-x}
    \pgfplotsautonode[preferred pos=0.35,sloped=true]{$y=4-x$};
\end{axis}
\end{tikzpicture}
\end{document}

.