[Tex/LaTex] How to use colors from a pgfplots colormap in own draw

pgfplotstikz-pgf

Let's consider a set of ellipses (or say a signal), here 3, but in general that might be a huge array of 64x64

\documentclass{standalone}
\usepackage{tikz,pgfplots}

\usetikzlibrary{calc}

\begin{document}
    \begin{tikzpicture}
            \draw[rotate around={-20:(0,0)},black] (0,0) ellipse (.5 and .25);
            \draw[rotate around={0:(1,0)},black] (1,0) ellipse (.45 and .3);
            \draw[rotate around={20:(2,0)},black] (2,0) ellipse (.4 and .35);
    \end{tikzpicture}
\end{document}

And I further have values from 0 to 1 for each ellipse indicating a color, that is i would like to replace the black by some color from a colormap, maybe the /pgfplots/colormaps/hue colormap. Let these values for example be

 {0.2, 0.6, 0.3}

The I would like to read the colormap values at these points and use them to fill (or draw) the ellipses.

So for short the question is: How can I access a specific color in a pgfplots-colormap?

Best Answer

As with the release of PGFPlots v1.13 you can use the new keys color of colormap or index of colormap to get easy access to the colors of the colormap. See section 4.7.6 pages 192f in the manual.

Here your code again using the first mentioned feature

\documentclass{standalone}
\usepackage{pgfplots}
    \usepgfplotslibrary{colormaps}
    \pgfplotsset{compat=1.11}
    \tikzset{
        ellC/.style={
            color of colormap={#1},
            draw=.!80!black,
            fill=.!80!white,
        },
    }
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            hide axis,
            colormap/hsv,
            xmin=-2.5, xmax=2.5,
            ymin=-.5, ymax=.5,
            axis equal,
        ]
            \draw[ellC=200, rotate around={-20:(0,0)}] (0,0) ellipse (.5  and .25);
            \draw[ellC=600, rotate around={0:(1,0)}]   (1,0) ellipse (.45 and .3);
            \draw[ellC=300, rotate around={20:(2,0)}]  (2,0) ellipse (.4  and .35);
        \end{axis}
    \end{tikzpicture}
\end{document}

Related Solutions

[Tex/LaTex] Rotate a node but not its content: the case of the ellipse decoration

The source of the difficulty is that ellipses are constructed in a particular way in TikZ. They are paths that start from the x-axis and proceed counter-clockwise around their centre. The vast majority of the time, the exact parametrisation doesn't matter. You appear to have found the one situation where it does!

In the actual question, you only want to be able to mirror the ellipse, and so draw it starting from the negative x-axis (the title of the question suggests a more flexible approach). That's actually not too hard since we can exploit the symmetry of the ellipse. The key is to provide it with a negative x-radius, since then it will start from the negative x-axis (and proceed clockwise, but we could correct for that by negating the y-radius as well). To do this, we interrupt the call from the node shape to the drawing command and flip the sign of the x-radius. The simplest way to do this is to redefine the \pgfpathellipse macro to do the negation and then call the original macro. The following code does this.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations,shapes,decorations.markings}
\makeatletter
\let\origpgfpathellipse=\pgfpathellipse
\def\revpgfpathellipse#1#2#3{%
  #2%
  \pgf@xa=-\pgf@x
  \origpgfpathellipse{#1}{\pgfqpoint{\pgf@xa}{0pt}}{#3}}
\makeatother

\tikzset{
  reversed ellipse/.style={
    ellipse,
    reverse the ellipse%
  },
  reverse the ellipse/.code={
    \let\pgfpathellipse=\revpgfpathellipse
  }
}
\begin{document}
\begin{tikzpicture}
\node[ellipse,
  draw,
  postaction={
    decorate,
    decoration={
      markings,
      mark=at position 1 with {
        \arrow[line width=5pt,blue]{>}
      }
    }
  }
] at (0,0) {hello world};

\node[reversed ellipse,
  draw,
  postaction={
    decorate,
    decoration={
      markings,
      mark=at position 1 with {
        \arrow[line width=5pt,blue]{>}
      }
    }
  }
] at (0,-2) {hello world};
\end{tikzpicture}

\end{document}

Here's the result:

rotated ellipse

(the arrow got clipped, but you can see where it lies)

[Tex/LaTex] pgfplots sample colors from colorbar

You can make a colour from the colormap available as mapped color by calling the macro \pgfplotscolormapdefinemappedcolor{<value>}, where <value> is a number between 0 and 1000 that is mapped linearly into the current color map.

In order to use different colours for different plots, this macro needs to be called within execute at begin plot visualization, otherwise all the plots end up with the same colour:

\documentclass[border=1mm]{standalone}
\usepackage{pgfplots}
%\usepgfplotslibrary{colormaps}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
    colormap/hot,
    domain=0:360,
    colorbar sampled,
    colorbar style={samples=6},
    point meta min=0,
    point meta max=10
]

\pgfplotsinvokeforeach{1,...,5}{
\addplot [
    execute at begin plot visualization={%
        \pgfplotscolormapdefinemappedcolor{\numexpr#1*200-100\relax}
    },
    mapped color,
    thick
] {#1*sin(x)};
}
\end{axis}
\end{tikzpicture}
\end{document}

Best Answer

Related Solutions

[Tex/LaTex] Rotate a node but not its content: the case of the ellipse decoration

[Tex/LaTex] pgfplots sample colors from colorbar

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