Why does PGFplots plot more than the desired range in a 3D plot?

Question

I would like to plot a function in the range [-2, 2], but PGFplots keeps plotting a little bit more:

enter image description here

Why is that the case and how do I fix it?

Code

\documentclass[varwidth=true, border=2pt]{standalone}

\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{pgfplots}
\pgfplotsset{compat=1.11}
\usepackage{tikz}

\begin{document}
\pgfplotsset{
    colormap={whitered}{
        color(0.000cm)=(blue!50!black);
        color(0.125cm)=(blue);
        color(0.250cm)=(SkyBlue);
        color(0.375cm)=(SkyBlue);
        color(0.500cm)=(SpringGreen);
        color(0.625cm)=(yellow);
        color(0.750cm)=(yellow);
        color(0.875cm)=(red);
        color(1.000cm)=(red!50!black)
    }
}
\begin{tikzpicture}
    \begin{axis}[
        domain=-2:2,
        view={0}{90},
        axis background/.style={fill=white},
    ]
        \addplot3[surf,
                  samples=50,
                  shader=interp,
                  colormap name=whitered]
                {x/exp(x^2+y^2)};
        % \addplot3[contour gnuplot={number=15, labels=false},
        %           very thick,
        %           samples=30] 
        %         {x/exp(x^2+y^2)};
        \addplot3[blue,
                  quiver={
                          u={(1-2*x^2)*exp(-x^2-y^2)},
                          v={-2*x*y*exp(-x^2-y^2)},
                          scale arrows=0.3,
                         },
                  -stealth,samples=15]
                {x/exp(x^2+y^2)};
    \end{axis}
\end{tikzpicture}
\end{document}

This is again a try to improve the quality of the images on Wikipedia. The original image is File:Gradient of a Function.tif and my image is File:3d-gradient-colored.svg — Martin Thoma, Feb 08 '15 at 11:27

score 10 · Accepted Answer · edited Apr 13 '17 at 12:35

This happens because the lengths of the quiver arrows are taken into account when calculating the axis range. The arrow that starts at (-2,0) has a non-zero length, the tip lies slightly to the left, so the axis range is increased so the full arrow is visible.

To limit the axis range to the function domain, set xmin=-2, xmax=2, ymin=-2, ymax=2.

I would also recommend setting axis equal image to get the same scaling along the x and y axis, and using the approach from Quiver plot with pgfplots: scale arrowheads with arrowsize for also scaling the arrowhead sizes, not just the lengths:

\documentclass[varwidth=true, border=2pt]{standalone}

\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{pgfplots}
\pgfplotsset{compat=1.11}
\usetikzlibrary{arrows.meta}

\begin{document}
\pgfplotsset{
    colormap={whitered}{
        color(0.000cm)=(blue!50!black);
        color(0.125cm)=(blue);
        color(0.250cm)=(SkyBlue);
        color(0.375cm)=(SkyBlue);
        color(0.500cm)=(SpringGreen);
        color(0.625cm)=(yellow);
        color(0.750cm)=(yellow);
        color(0.875cm)=(red);
        color(1.000cm)=(red!50!black)
    }
}
\begin{tikzpicture}
    \begin{axis}[
        domain=-2:2,
        view={0}{90},
        axis background/.style={fill=white},
        xmin=-2, xmax=2,
        ymin=-2, ymax=2,
        axis equal image,
        point meta rel=per plot
    ]
        \addplot3[surf,
                  samples=15,
                  shader=interp,
                  colormap name=whitered]
                {x/exp(x^2+y^2)};
        \addplot3[blue,
        point meta={
            sqrt(
                ((1-2*x^2)*exp(-x^2-y^2))^2+
                (-2*x*y*exp(-x^2-y^2))^2
            )        
        },
                  quiver={
                          u={(1-2*x^2)*exp(-x^2-y^2)},
                          v={-2*x*y*exp(-x^2-y^2)},
                          scale arrows=0.3,
                          every arrow/.append style={%
                             -{Latex[scale length={max(0.01,\pgfplotspointmetatransformed/1000)}]},
                             },
                         },
                         samples=15]
                {x/exp(x^2+y^2)};
    \end{axis}
\end{tikzpicture}
\end{document}

Why does PGFplots plot more than the desired range in a 3D plot?

Code

1 Answers1