Non-uniform colormap

Question

I am trying to use non-uniform colormap and I get this error:

Package pgfplots Error: Sorry, non-uniform colormaps are only partially implemented, yet: the provided points must be multiples of the mesh width h=0.2275pt (but I found one with H/h = 0.68292pt/0.2275pt = 3+-0.0019 which is no integer). Perhaps it helps to provide the mesh widths as argument as in {<name>}{[1cm] <color arguments>}? The error occured near `rgb255(0.032cm)=(0,0,74)'.

I do not quite unrestand the error and how to workaround to prevent it. Can you help me with this? Below is the MWE.

I wo

\documentclass{minimal}
\usepackage{pgfplots}
\usepackage{filecontents}

\begin{filecontents}{surfdata.dat}
 0 0 0.8
 1 0 0.56
 2 0 0.5
 3 0 0.75

 0 2 0.6
 1 2 0.3
 2 2 0.21
 3 2 0.3

 0 4 0.68
 1 4 0.22
 2 4 0.25
 3 4 0.4

 0 6 0.7
 1 6 0.5
 2 6 0.58
 3 6 0.9
\end{filecontents}


\pgfplotsset{colormap/special/.style={colormap={iron}{rgb255(0cm)=(0,0,0) rgb255(0.008cm)=(0,0,30) rgb255(0.032cm)=(0,0,74) rgb255(0.072cm)=(13,0,117) rgb255(0.123cm)=(52,0,142) rgb255(0.182cm)=(93,0,154) rgb255(0.245cm)=(139,0,157) rgb255(0.308cm)=(176,1,152) rgb255(0.368cm)=(197,12,140) rgb255(0.418cm)=(211,32,113) rgb255(0.457cm)=(222,50,74) rgb255(0.482cm)=(227,61,38) rgb255(0.49cm)=(228,65,29) rgb255(0.55cm)=(237,91,6) rgb255(0.611cm)=(244,115,0) rgb255(0.725cm)=(253,168,0) rgb255(0.794cm)=(254,200,0) rgb255(0.845cm)=(255,220,18) rgb255(0.9cm)=(255,238,87) rgb255(1cm)=(255,255,255)}}}

\begin{document}

\begin{tikzpicture}
\begin{axis}[colormap/special,colorbar horizontal,point meta min=0,point meta max=1]
  \addplot3[surf,shader=interp] file {surfdata.dat};
\end{axis}
\end{tikzpicture}

\end{document}

Answer 1

To do so the PGFPlots manual (v1.13) states in section 4.7.6 on page 187:

It is also possible to provide non-uniform distances between the different colors - if all single positions can be projected onto a uniform grid. PGFPlots will perform this interpolation automatically:

[code example]

In this last example, the mesh width has been provided explicitly and PGFPlots interpolates the missing grid points on its own. It is an error if the provided positions are no multiple of the mesh width.

Said that, here is a working example of your colormap, where I only added [1pt] as mesh width, multiplied the distances by 1000 and changed the cms to pts.

(It seems that this is some kind of a high resolution colormap of colormap/thermal from the colormaps library, right?)

\begin{filecontents}{surfdata.dat}
 0 0 0.8
 1 0 0.56
 2 0 0.5
 3 0 0.75

 0 2 0.6
 1 2 0.3
 2 2 0.21
 3 2 0.3

 0 4 0.68
 1 4 0.22
 2 4 0.25
 3 4 0.4

 0 6 0.7
 1 6 0.5
 2 6 0.58
 3 6 0.9
\end{filecontents}
\documentclass[border=2mm]{standalone}
\usepackage{filecontents}
\usepackage{pgfplots}
    \pgfplotsset{
        colormap/special/.style={
            % provide a mesh width for a non-uniform colormap
            colormap={iron}{[1pt]
                rgb255(0pt)=(0,0,0)
                rgb255(8pt)=(0,0,30)
                rgb255(32pt)=(0,0,74)
                rgb255(72pt)=(13,0,117)
                rgb255(123pt)=(52,0,142)
                rgb255(182pt)=(93,0,154)
                rgb255(245pt)=(139,0,157)
                rgb255(308pt)=(176,1,152)
                rgb255(368pt)=(197,12,140)
                rgb255(418pt)=(211,32,113)
                rgb255(457pt)=(222,50,74)
                rgb255(482pt)=(227,61,38)
                rgb255(490pt)=(228,65,29)
                rgb255(550pt)=(237,91,6)
                rgb255(611pt)=(244,115,0)
                rgb255(725pt)=(253,168,0)
                rgb255(794pt)=(254,200,0)
                rgb255(845pt)=(255,220,18)
                rgb255(900pt)=(255,238,87)
                rgb255(1000pt)=(255,255,255)
            }
        }
    }
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            colormap/special,
            colorbar horizontal,
            point meta min=0,
            point meta max=1,
        ]
            \addplot3 [surf,shader=interp] table {surfdata.dat};
        \end{axis}
    \end{tikzpicture}
\end{document}