[Tex/LaTex] How to use “axis cs” of pgfplots with mathematical expression

pgfplots

Inside the environment axis I can specify some points with axis cs. Up to now I am only able to use specific coordinates like 1,1. But I want to know how to use a mathematical expression like ln(3),2.

Here is an example:

\documentclass{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage{pgfplots}
\pgfplotsset{compat=1.4}
\begin{document}
\begin{tikzpicture}
\begin{axis}[axis equal image,
width=10cm,
axis lines=middle,
axis line style={-latex},
xtick=\empty,
ytick=\empty,
ymin=-2.5, ymax=2.5,
xmin=-3.4,xmax=3.3,
clip =false,
xlabel={$x$},ylabel={$y$},
every axis x label/.append style={anchor=north},
every axis y label/.append style={anchor=east},
]

\foreach \c in {-2,-1,-0.5,0.5,1,2}
  \addplot[domain=-3:3,samples=200,restrict y to domain=-2:2]{ln(\c*x)};

%ln(1*3)1,098612289
\node[anchor=west] at (axis cs:3,1.099) {$c=1$};
%\node[anchor=west] at (axis cs:3,ln(3)) {$c=1$};
\end{axis}
\end{tikzpicture}
\end{document}

At the moment I am using:

%ln(1*3)1,098612289
\node[anchor=west] at (axis cs:3,1.099) {$c=1$};

but I would prefer a more automatic way

\node[anchor=west] at (axis cs:3,ln(3)) {$c=1$};

Best Answer

You can say

\pgfmathlog{3}\let\mylog\pgfmathresult
\node[anchor=west] at (axis cs:3,{\mylog}) {$c=1$};

Not exactly the same, but I guess that performing complex calculations when looking for coordinates is not easy.

Related Solutions

[Tex/LaTex] pgfplots and mathematical expression with coordinates list

From the manual about coordinate lists

You should only use this input format if you have short diagrams and you want to provide mathematical expressions for each of the involved coordinates. Any data plots are typically easier to handle using a table format and \addplot table.

But if you really have to, you have to provide the coordinate transformations manually for all plots in the axis environment hence it wouldn't be ideal because the axes needs to be setup: Here I fake a linear plot with a squared input

\documentclass{standalone}
\usepackage{pgfplots}
\begin{document}
  \begin{tikzpicture}
    \begin{axis}[ 
      axis x line=bottom,
      axis y line=left,
      ultra thick,
      y coord trafo/.code={\pgfmathparse{sqrt(#1)}},
      y coord inv trafo/.code={\pgfmathparse{pow(#1,2)}}
    ] 
    \addplot coordinates {(0,0)(1,1)(2,4)(3,9)(4,16)(5,25)};
  \end{axis}
\end{tikzpicture}
\end{document}

enter image description here

See section 4.22 for more information about these transformations

PGFPlots – How to Hide Empty Ybars in Bar Charts

You can use

 y filter/.expression={y==0 ? nan : y}

in the options of \addplot.

\documentclass{article}

% ---------------------------------- tikz
\usepackage{pgfplots}          % to print charts
\pgfplotsset{compat=1.12}

\begin{document}

\begin{figure}
  \centering
  \begin{tikzpicture}
    \begin{axis} [
      % general
      ybar,
      scale only axis,
      height=0.5\textwidth,
      width=1.2\textwidth,
      ylabel={\# Dots},
      nodes near coords,
      xlabel={Variation},
      xticklabel style={
        rotate=90,
        anchor=east,
      },
      %enlarge x limits={abs value={3}},
      ]
      \addplot+[y filter/.expression={y==0 ? nan : y}] table [
        x=grade,
        y=value,
      ] {
grade   value
-11 0
-10 0
-9  0
-8  0
-7  0
-6  0
-5  3
-4  1
-3  2
-2  15
-1  11
0   179
1   8
2   1
3   0
4   1
5   2
6   0
7   0
8   0
9   0
10  0
11  0
      };
    \end{axis}
  \end{tikzpicture}
\end{figure}
\end{document}

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