How can I draw a fancy "proxy" legend line with pgfplots?

Question

I have a couple of curves I am drawing in an axis, but one of them is a circle that is drawn with a "fancy" line (I just overlay a thick black curve with a slightly thinner white dashed curve). Is there a way I can fake a legend entry to match?

I have looked in the pgfplots manual and I see \addlegendimage and \addlegendentry but can't think of how to use it except for a rudimentary simple line:

\documentclass[tikz, border=2mm]{standalone}
\usepackage{pgfplots}
\usepackage{sansmath}
\pgfplotsset{compat=1.17}
% pgfplots package manual at https://ctan.org/pkg/pgfplots?lang=en
\begin{document}
\begin{tikzpicture}
\begin{axis}[
    axis equal,
    width=10cm, height=6cm,
    font=\sffamily,
    ticklabel style = {font=\sansmath\sffamily},
    xmin=-0.7,xmax=0, xlabel={$x$}, xtick={-1,-0.9,...,0}, minor xtick={-1,-0.95,...,0}, 
    ymin=0.7,ymax=1.0, ylabel={$y$}, ytick={0, 0.1,...,1}, minor ytick={0, 0.05,...,1},
    samples=500,domain=-1:0,
    grid=both,
    legend pos = south east,
    legend cell align = left,
    title={\large shapes on axis}]
% I want to add a legend saying "circle"
  \draw [black, line width = 0.7mm] (0,0) circle [radius=1.0];
  \draw [white, line width = 0.5mm, dash pattern = on 5pt off 5pt] (0,0) circle [radius=1.0];
  \addlegendimage{black, line width=0.7mm};
\addplot[blue, line width = 0.3mm]({x}, {1-0.5xx} );
% I want to add a legend saying "line"
  \draw [green!50!black, line width=0.3mm] (-0.6,1) -- (-0.4,0.95);
  \addlegendimage{green!50!black, line width=0.3mm};
\legend {circle, quadratic curve, line};
\end{axis}
\end{tikzpicture}
\end{document}

How can I make the circle legend have the same line pattern?

Yoou should be able to just use the same style. But in you case you draw two separate plots to obtain the fancy style. If you come up with a single style that daws you graph, then it should work. — Peter Grill, Sep 15 '20 at 04:05
Yeah, sure, but I don't know how to do that with a single style. I like the bent-striped-soda-straw look much better than a single line. There's no way to draw two different legend lines in the same spot? (without having to recreate the function of pgfplots legend) — Jason S, Sep 15 '20 at 04:08
One way would be to use a decoration on the path, but perhaps there is an easier way. — Peter Grill, Sep 15 '20 at 04:18

score 4 · Accepted Answer · answered Sep 15 '20 at 06:58

Using option postaction (or preaction), the two \draws can be merged to one. These two options are documented in pgfmanual, sec. 15.10 Doing Multiple Actions on a Path in v3.1.5b.

Similar to this answer, an example using postaction is (note the option draw used in postactions={...})

\documentclass[tikz, border=2mm]{standalone}
\begin{document}
\begin{tikzpicture}
  \draw
      [postaction={draw, white, dash pattern=on 4pt off 4pt, dash phase=4pt, thick}]
      [black, ultra thick] 
      (0,0) rectangle (3,2);
\end{tikzpicture}
\end{document}

By wrapping all the options in a new style double colors, your example can be drawn by (note the dash phase=-4pt added to \addlegendimage[...])

\documentclass[tikz, border=2mm]{standalone}
\usepackage{pgfplots}
\usepackage{sansmath}
\pgfplotsset{compat=1.17}
\tikzset{
  double colors/.style={
    postaction={draw, white, line width = 0.5mm, dash pattern = on 5pt off 5pt},
    black, line width = 0.7mm
  }
}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
    axis equal,
    width=10cm, height=6cm,
    font=\sffamily,
    ticklabel style = {font=\sansmath\sffamily},
    xmin=-0.7,xmax=0, xlabel={$x$}, xtick={-1,-0.9,...,0}, minor xtick={-1,-0.95,...,0}, 
    ymin=0.7,ymax=1.0, ylabel={$y$}, ytick={0, 0.1,...,1}, minor ytick={0, 0.05,...,1},
    samples=500,domain=-1:0,
    grid=both,
    legend pos = south east,
    legend cell align = left,
    title={\large shapes on axis}]
% I want to add a legend saying "circle"
  \draw [double colors] (0,0) circle [radius=1.0];
  \addlegendimage{double colors, dash phase=-4pt, line width=0.7mm};
\addplot[blue, line width = 0.3mm]({x}, {1-0.5xx} );
% I want to add a legend saying "line"
  \draw [green!50!black, line width=0.3mm] (-0.6,1) -- (-0.4,0.95);
  \addlegendimage{green!50!black, line width=0.3mm};
\legend {circle, quadratic curve, line};
\end{axis}
\end{tikzpicture}
\end{document}

Thanks! So I guess the takeaway here is that it is difficult (impossible?) to draw custom art in a legend; the cleaner approach in pgfplots is to encapsulate it as a style that can be applied to the line that the legend-handling logic can just use. — Jason S, Sep 15 '20 at 18:15
@JasonS Ah I just find the option legend image code/.code={...}, which can be used to set arbitrary drawing code for all legends. By default this option is set by legend line to \draw plot coordinates {...};. Not sure if there is an easy way to only change drawing code for specific legends. — muzimuzhi Z, Sep 15 '20 at 20:55

How can I draw a fancy "proxy" legend line with pgfplots?

1 Answers1