Log color bar/meta data in PGFPLOT

Question

I'm trying to create a 2D plot with a color map using pgfplots. I've managed to get the overall behaviour that I want, but I would like the color map to be logarithmic, rather than linear. The best I have managed is to use the meta expr key, but then the color bar shows 1-4 instead of 10^1 through 10^4. Below is my working example.

\begin{tikzpicture}
\begin{semilogyaxis}[colorbar,
xlabel = create energy (keV),
ylabel = deposit energy (kev),
]
\pgfplotstableread{../endep_log_au_30_0_0.csv}\mytable %
\addplot+[scatter, only marks, scatter src=explicit, mark=square]
 table[x=createEn,y=depEn,meta expr=ln(\thisrow{tot})/ln(10)] {\mytable};
\end{semilogyaxis}
\end{tikzpicture}

Is there a way to modify how the labels appear on the colorbar or set it to be logarithmic? Is there a different way I should be approaching this problem?

(Unfortunately I cannot post an image as a new user)

Answer 1

The meta expr approach is a good one. In order to get the labels on the colorbar right, you can set \yticklabel=\pgfmathparse{10^\tick}\pgfmathprintnumber{\pgfmathresult} for the colorbar -- colorbars are full axis environments, so they have all the options available to "normal" plots. You can set the options for the colorbar using colorbar style={<options>}:

\documentclass{article}

\usepackage{pgfplots}
\usepackage{pgfplotstable}

\begin{document}

% Define new pgfmath function for the logarithm to base 10 that also works with fpu library
\pgfmathdeclarefunction{lg10}{1}{%
    \pgfmathparse{ln(#1)/ln(10)}%
}

\pgfplotstableread{
createEn depEn tot
0 5 1
0.1 20 10
0.5 100 100
2 10300 1000
}\mytable %


\begin{tikzpicture}
\begin{semilogyaxis}[
    xlabel = create energy (keV),
    ylabel = deposit energy (keV),
    colorbar,
    colorbar style={yticklabel=\pgfmathparse{10^\tick}\pgfmathprintnumber\pgfmathresult}
]

\addplot+[scatter, only marks, scatter src=explicit, mark=square]
 table[meta expr=lg10(\thisrow{tot})] {\mytable};
\end{semilogyaxis}
\end{tikzpicture}


\end{document}