Making a simple legend on tikz

Question

I'm writing a document that requires very few actual plots and I'm happy with the aesthetic of tikz. However, I'm having trouble finding a way to make a simple legend on tikz. I know there's a way with the data visualization library, but it seems far too convoluted for simple graphs. Some people suggested using matrices, which turned out to be pretty much what I wanted, like this:

\documentclass[11pt]{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\draw[->] (-1,0) -- (8,0)  node[right]{$x$};
\draw[->] (0,-2) -- (0,2)  node[above]{$y$};
\draw[green,samples=100,domain=-1:8] plot(\x,{sin(deg(\x))});
\draw[red,samples=100,domain=-1:8] plot(\x,{cos(deg(\x))});
\draw[blue] (0,0)--(pi/2,1)--(3*pi/2,-1)--(5*pi/2,1);
\matrix [draw, above left] at (8,-2) {
  \node[green,font=\tiny] {$\sin x$}; \\
  \node[red,font=\tiny] {$\cos x$}; \\
  \node[blue,font=\tiny] {Lines}; \\
};
\end{tikzpicture}
\end{document}

However, I want the legend to show the draw style of the graphs. Like what we can see on the tikz manual:

Can I do that with matrices? I would also want the legend to be aligned to the left, right now it's to the right, but no idea how to change it.

Answer 1

I also prefer pfgplots but for completeness here is a method using pics.

\documentclass[11pt]{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[pics/legend entry/.style={code={%   
        \draw[pic actions] 
        (-0.5,0.25) sin (-0.25,0.4) cos (0,0.25) sin (0.25,0.1) cos (0.5,0.25);}}]
\draw[->] (-1,0) -- (8,0)  node[right]{$x$};
\draw[->] (0,-2) -- (0,2)  node[above]{$y$};
\draw[green!70!black,samples=100,domain=-1:8] plot(\x,{sin(deg(\x))});
\draw[red,samples=100,domain=-1:8] plot(\x,{cos(deg(\x))});
\draw[blue] (0,0)--(pi/2,1)--(3*pi/2,-1)--(5*pi/2,1);
\matrix [draw, above left] at (8,-2) {
 \pic[green!70!black]{legend entry}; &  \node[green!70!black,font=\tiny] {$\sin x$}; \\
 \pic[red]{legend entry}; &  \node[red,font=\tiny] {$\cos x$}; \\
 \pic[blue]{legend entry}; &  \node[blue,font=\tiny] {Lines}; \\
};
\end{tikzpicture}
\end{document}

To align the text nodes you can use these tricks.

\documentclass[11pt]{article}
\usepackage{tikz}
\usepackage{eqparbox}
\begin{document}
\newbox\eqnodebox
\tikzset{lequal size/.style={execute at begin
    node={\setbox\eqnodebox=\hbox\bgroup},
    execute at end node={\egroup\eqmakebox[#1][l]{\copy\eqnodebox}}},
    lequal size/.default=A,}
\begin{tikzpicture}[pics/legend entry/.style={code={%   
        \draw[pic actions] 
        (-0.5,0.25) sin (-0.25,0.4) cos (0,0.25) sin (0.25,0.1) cos (0.5,0.25);}}]
\draw[->] (-1,0) -- (8,0)  node[right]{$x$};
\draw[->] (0,-2) -- (0,2)  node[above]{$y$};
\draw[green!70!black,samples=100,domain=-1:8] plot(\x,{sin(deg(\x))});
\draw[red,samples=100,domain=-1:8] plot(\x,{cos(deg(\x))});
\draw[blue] (0,0)--(pi/2,1)--(3*pi/2,-1)--(5*pi/2,1);
\matrix [draw, above left] at (8,-2) {
 \pic[green!70!black]{legend entry}; &  \node[lequal size,green!70!black,font=\tiny] {$\sin x$}; \\
 \pic[red]{legend entry}; &  \node[lequal size,red,font=\tiny] {$\cos x$}; \\
 \pic[blue]{legend entry}; &  \node[lequal size,blue,font=\tiny] {Lines}; \\
};
\end{tikzpicture}
\end{document}

Answer 2

To get a consistent look, I suggest to never use pure TikZ for plots, but PGFPlot that is build on top of TikZ.

\documentclass[11pt]{article}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[%
  samples=100,
  domain=-1:8,
  xmin=-1, xmax=8,
  ymin=-2, ymax=2,
  axis lines=middle,
  ticks=none,
  xlabel={$x$},
  ylabel={$y$},
  legend pos=south east,
  width=\textwidth,
  height=0.5*\textwidth]
\addplot[green] {sin(deg(\x))};
\addplot[red] {cos(deg(\x))};
\addplot[blue] coordinates{(0,0) (pi/2,1) (3*pi/2,-1) (5*pi/2,1)};
\addlegendentry{$sin(x)$}
\addlegendentry{$cos(x)$}
\addlegendentry{Lines}
\end{axis};
\end{tikzpicture}
\end{document}