Changing arrowheads size in pgfplots axes

Question

I need to change the default axes arrowheads size of pgfplots when using the option axis lines=center. Following this answer, I tried this code:

\documentclass{standalone}
\usepackage{pgfplots}
\usetikzlibrary{arrows, decorations.markings}

\begin{document}
\begin{tikzpicture}
\tikzstyle{myaxis} = [decoration={markings, mark=at position 1 with {\arrow[ultra thick]{latex'}}},
                      postaction={decorate}, shorten >=0.4, line width=0.02pt];
\begin{axis}[axis lines=center, axis line style={myaxis}]
\end{axis}
\end{tikzpicture}
\end{document}

but it doesn't work as I would expect, since the axes lines aren't drawn at all.

Could you help me? Thanks,

Luigi

Answer 1

Customising axis lines is a bit unintuitive: For one, the decoration to be used in the postaction needs to be specified within the postaction. And applying a postaction to the axes when using axis lines=left (instead of axis lines=middle, for example) requires the use of the every path/.style (similar to what was done in How to specify a name path for the axis in PGFplots). To avoid an infinite recursion in this case, you need to clear the postaction once it's been executed for the first time. This can be done using the nomorepostaction key described in Applying a postaction to every path in TikZ.

Here's your axis generated using

\begin{axis}[
    axis lines=middle,
    axis line style={my axis}
]
\addplot coordinates {(-0.1,-0.2) (1.2,1.2)};
\end{axis}

And here's the complete code:

\documentclass[border=5mm]{standalone}
\usepackage{pgfplots}
\usetikzlibrary{arrows, decorations.markings}

\begin{document}
\begin{tikzpicture}

\makeatletter
\tikzset{
    nomorepostaction/.code=\makeatletter\let\tikz@postactions\pgfutil@empty, % From http://tex.stackexchange.com/questions/3184/applying-a-postaction-to-every-path-in-tikz/5354#5354
    my axis/.style={
        postaction={
            decoration={
                markings,
                mark=at position 1 with {
                    \arrow[ultra thick]{latex}
                }
            },
            decorate,
            nomorepostaction
        },
        thin,
        -, % switch off other arrow tips
        every path/.append style=my axis % this is necessary so it works both with "axis lines=left" and "axis lines=middle"
    }
}
\makeatother


\begin{axis}[
    axis lines=middle,
    axis line style={my axis}
]
\addplot coordinates {(-0.1,-0.2) (1.2,1.2)};
\end{axis}
\end{tikzpicture}
\end{document}

Answer 2

I tried this solution and I think it's better for me. I edited the definition of the latex' arrowhead so that you can specify directly the tip length or use the default value (8pt). So the arrowhead size is now no more dependent on the line width. Obviously you can use the same solution in plain TikZ.

\documentclass{standalone}
\usepackage{pgfplots}
\pgfarrowsdeclare{mylatex'}{mylatex'}
{
  \newdimen\len
  \len=\pgfgetarrowoptions{mylatex'}
  \pgfarrowsleftextend{-0.4\len}
  \pgfarrowsrightextend{0.6\len}
}
{
  \newdimen\len
  \len=\pgfgetarrowoptions{mylatex'}
  \pgfpathmoveto{\pgfqpoint{0.6\len}{0\len}}
  \pgfpathcurveto
  {\pgfqpoint{0.35\len}{0.05\len}}
  {\pgfqpoint{-0.1\len}{0.15\len}}
  {\pgfqpoint{-0.4\len}{0.375\len}}
  \pgfpathcurveto
  {\pgfqpoint{-0.15\len}{0.1\len}}
  {\pgfqpoint{-0.15\len}{-0.1\len}}
  {\pgfqpoint{-0.4\len}{-0.375\len}}
  \pgfpathcurveto
  {\pgfqpoint{-0.1\len}{-0.15\len}}
  {\pgfqpoint{0.35\len}{-0.05\len}}
  {\pgfqpoint{0.6\len}{0\len}}
  \pgfusepathqfill
}
\pgfsetarrowoptions{mylatex'}{8pt}
\pgfkeys{/tiplen/.default=8pt, /tiplen/.code={\pgfsetarrowoptions{mylatex'}{#1}}}

\begin{document}
\begin{tikzpicture}
\begin{axis}[axis lines=left, axis line style={-mylatex'}]
\end{axis}
\end{tikzpicture}
\begin{tikzpicture}
\begin{axis}[axis lines=middle, axis line style={-mylatex', /tiplen=4pt}]
\addplot [only marks] coordinates {(0,0) (0.5,0.5)};
\draw [-mylatex', /tiplen=1cm] (axis cs: 0.1,0.1) -- (axis cs: 0.4,0.4);
\end{tikzpicture}
\end{document}