Y-axis square root scaling

Question

I am trying to create a boxplot of my data in pgfplots with square-root scaling of Y axis, due to the nature of data.

This is my code:

\documentclass{article} 
\usepackage{pgfplots}
\usetikzlibrary{pgfplots.statistics}
\pgfplotsset{compat=1.17}
\usepackage{filecontents}
\begin{filecontents}{file.csv}
a,b,c,d,e
0.0,2.842170943040401e-14,0.0,0.02581787109375,1.8994026184082031
0.0,0.3329588710026492,0.75,0.02837371826171875,0.4414405822753906
0.0,0.15510184685969364,0.375,0.1663665771484375,0.199951171875
0.0,0.000375662574413127,0.0,0.2701606750488281,0.8085689544677734
0.0,1.4210854715202004e-14,0.0,0.03917694091796875,0.25116539001464844
\end{filecontents}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
    boxplot/draw direction=y,
    ymin=0., ymax=64.,
    y coord trafo/.code={\pgfmathparse{sqrt{#1}}},  % Y-axis forward transform
    y coord inv trafo/.code={\pgfmathparse{#1^2}},  % Y-axis backward transform
    table/row sep=newline,
    table/col sep=comma
]
\addplot+[boxplot={draw position=1}, fill, draw=black, solid, mark=none] table [y=a] {file.csv};
\end{axis}
\end{tikzpicture} 
\end{document}

I am getting an ambiguous error message

This is pdfTeX, Version 3.141592653-2.6-1.40.22 (TeX Live 2021) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./sqrt_plot.tex
LaTeX2e <2020-10-01> patch level 4
L3 programming layer <2021-02-18>
(/usr/local/texlive/2021/texmf-dist/tex/latex/base/article.cls
Document Class: article 2020/04/10 v1.4m Standard LaTeX document class
(/usr/local/texlive/2021/texmf-dist/tex/latex/base/size10.clo))
(/usr/local/texlive/2021/texmf-dist/tex/latex/pgfplots/pgfplots.sty
(/usr/local/texlive/2021/texmf-dist/tex/generic/pgfplots/pgfplots.revision.tex)
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(/usr/local/texlive/2021/texmf-dist/tex/latex/graphics/keyval.sty)
(/usr/local/texlive/2021/texmf-dist/tex/latex/graphics/graphics.sty
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(/usr/local/texlive/2021/texmf-dist/tex/latex/graphics-cfg/graphics.cfg)
(/usr/local/texlive/2021/texmf-dist/tex/latex/graphics-def/pdftex.def)))
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x
(/usr/local/texlive/2021/texmf-dist/tex/generic/pgf/utilities/pgfutil-common-li
sts.tex))
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./sqrt_plot.tex:26: Missing \endcsname inserted.
<to be read again> 
                   \relax 
l.26 ... solid, mark=none] table [y=b] {file.csv};
?

It seems to be the problem with the coordinate scaling. However, when I replace the \addplot with following, it works.

\addplot { x^2 };  % this works

The problem must be related to, when coord trafo is combined with plotting data from CSV.

Can anybody help me, please?

As an aside, you can visualize sqrt functionality also on a logarithmic scale … — MS-SPO, Feb 21 '24 at 15:44
I wonder if you could do tis as a parametric plot? t=x*x and y=sqrt(t). You can use a list of tick marks matching the t values instead of the y vallues. — John Kormylo, Feb 21 '24 at 15:46
@MS-SPO, it is not the same, log is steeper. Also, my data contains zeros. — Martin Benes, Feb 21 '24 at 15:52
@JohnKormylo You mean convert the data to square-roots and then "fake" the Y-axis tick labels? Understood, a bit clumsy though. — Martin Benes, Feb 21 '24 at 15:53
Yes. To a certain extent, the pgfplots tick marks are just as fake. — John Kormylo, Feb 21 '24 at 15:57
There is no sqrt{} function - it is sqrt(). The problem is not related to data from a file, but to boxplot. — hpekristiansen, Feb 21 '24 at 16:40
Do not use the filecontents package - it has bee deprecated since 2019 — hpekristiansen, Feb 21 '24 at 19:34

score 2 · Accepted Answer · answered Feb 21 '24 at 17:53

At some point the boxplot uses y coord trafo with something that is not a number. -otherwise it seems to work. I have copied isPGFNumber from here: https://tex.stackexchange.com/a/463819/8650 and use it to test if #1 is a number before doing the conversion.

A working boxplot on a square-root scale:

\documentclass[tikz, border=1cm]{standalone} 
\usepackage{pgfplots}
\usetikzlibrary{pgfplots.statistics}
\newcommand{\isPGFNumber}[3]{% \isPGFNumber{<Number>}{<True>}{<False>}
    \begingroup
    \pgfkeys{/pgf/fpu/handlers/invalid number/.code={}}%
    \pgfmathfloatparsenumber{#1}%
    \global\pgfmathfloatgetflagstomacro\pgfmathresult\pgfFloatParseFlag%
    \endgroup
    \ifnum\pgfFloatParseFlag=3%
        #3%
    \else%
        #2%
    \fi%
}
\pgfplotsset{compat=1.18}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
ymin=0., ymax=64.,
boxplot/draw direction=y,
y coord trafo/.code={\isPGFNumber{#1}{\pgfmathparse{sqrt(#1)}}{}},
y coord inv trafo/.code={\pgfmathparse{#1^2}},
]
\addplot+[boxplot]  table[row sep=\\,y index=0] {4\\ 4\\ 4\\ 36\\ 36\\ 36\\};
\addplot {x^2};
\end{axis}
\end{tikzpicture} 
\end{document}

Y-axis square root scaling

1 Answers1