\documentclass{standalone}

\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepackage{amsmath}

\begin{document}


\begin{tikzpicture}
\begin{axis}[
legend cell align=left,
legend style={legend pos=north west}
]
  % added colours to plots
  \addplot[blue,domain=0:2] {x^2};
  \addplot[red,domain=0:4] {x};
  \addplot[orange,domain=0:4]{4-x};
  \legend{
    $\begin{aligned}[t]
    yy&=x^2\\[-2pt]
    y&=x\\
    yyy&=4-x
    \end{aligned}$,
    \strut % second legend entry
  }
\end{axis}
\end{tikzpicture}
\end{document}