\documentclass{standalone}

\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepackage{mathtools}
\newcommand{\LHS}[2][1.5em]{\hspace{#1}\mathllap{#2}}
\newcommand{\RHS}[2][1.5em]{\hspace{#1}\mathrlap{#2}}

\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{%
		$\LHS{yy}=x^2$ text,
		$\LHS{zty}=xxxx$ text,
		$\LHS{y}=x$ text,
	}
	\end{axis}
	\end{tikzpicture}
\end{document}