[Tex/LaTex] Add a legend title to the legend box with pgfplots

legendpgfplotstikz-pgf

I am using the awesome pgfplots package to draw some plots. I usually use \addlegendentry and get a nice default legend box. There are some options for legend box placement and layout (e.g. number of columns).

For some plots I would like to add a title inside the legend box.

Could not find the something about this in the manual. Does anyone know what is the way to customize the legend box?

Best Answer

You could perhaps use the \addlegendimage command, as in this discussion from the pgfplots-features mailing list.

An example, with a small \hspace hack to place the title more centered in the legend box:

\documentclass{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[legend pos=south east]
   \addlegendimage{empty legend}
   \addplot {sqrt(x)}; 
   \addplot {ln(x)}; 

   \addlegendentry{\hspace{-.6cm}\textbf{A title}}
   \addlegendentry{$\sqrt{x}$}
   \addlegendentry{$\ln{x}$}
\end{axis}
\end{tikzpicture}
\end{document}

enter image description here

For variety, here's a couple of more manual approaches. In each case the legend and title are separate entities, and the frame drawn afterwards. 

\documentclass[border=5mm,tikz]{standalone}
\usepackage{pgfplots}
\usetikzlibrary{fit}
\begin{document}
\begin{tikzpicture}
\begin{axis}[legend style={at={(rel axis cs:0.9,0.1)},above left,name=legend,draw=none}]
   \addplot {sqrt(x)}; 
   \addplot {ln(x)}; 

   \addlegendentry{$\sqrt{x}$}
   \addlegendentry{$\ln{x}$}
\end{axis}
   \node [above,font=\bfseries] (legendtitle) at (legend.north) {Legend title};
   \node [fit=(legendtitle)(legend),draw,inner sep=0pt] {};
\end{tikzpicture}
\begin{tikzpicture}
\begin{axis}[legend style={draw=none,legend to name=leg}]
   \addplot {sqrt(x)}; 
   \addplot {ln(x)}; 

   \addlegendentry{$\sqrt{x}$}
   \addlegendentry{$\ln{x}$}

   % place legend
   \node [above left] (L) at (rel axis cs:0.9,0.1) {\ref{leg}};
   % Add title
   \node [above,font=\bfseries] (LT) at (L.north) {Legend title};
   % if needed, add frame
   \node [fit=(L)(LT),draw,inner sep=0pt] {};
\end{axis}
\end{tikzpicture}
\end{document}
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