diaa

\documentclass{standalone}
 
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepackage{mathtools}
\newcommand{\LHS}[2][1.5em]{\hspace{#1}\mathllap{#2}}
\newcommand{\RHS}[2][1.5em]{\mathrlap{#2}\hspace{#1}} % moved \hspace after mathrlap
 
\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[red,domain=0:4] {x};
    \legend{%
        $\LHS{yy}=\RHS[2em]{x^2}$ text,
        $\LHS{zty}=\RHS[2em]{xxxx}$ text,
        $\LHS{y}=\RHS[2em]{x}$ text,
    }
    \end{axis}
    \end{tikzpicture}
\end{document}