pgfplots add xtick from array

Question

I have \points array and I want to add xtick and ytick on my axis automatically from it. How can I do that?

\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\def\points{{{-9,0.30},{-6,0.44},{-2,0.59},{3,1}}}
\pgfmathsetmacro{\len}{dim(\points)-1}
\begin{axis}
[
axis lines = center,
axis line style = thick,
% xtick= loop here for \points[\i][0]
% ytick= loop here for \points[\i][1] where \i=0,...,\len
clip=false
]
\addplot ({x},{x^2});
\end{axis}
\end{tikzpicture}
\end{document}

CarLaTeX · Answer 1 · 2021-11-08T06:27:03.267

2

No need to loop.

The problem is that the points of your MWE have nothing to do with your curve (leave out opacity=0 to see them), they are neither on the curve nor near the curve, so the graphical representation is not good.
I think it would be better with your actual points and curve.

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}
\begin{tikzpicture}
\def\points{(-9,0.30) (-6,0.44) (-2,0.59) (3,1)}
\begin{axis}[scale=1.5, 
    xtick=data,
    ytick=data,
    axis lines = center,
    axis line style = thick,
    clip=false
    ]
\addplot[only marks, 
    opacity=0] 
    coordinates {\points};
\addplot ({x},{x^2});
\end{axis}
\end{tikzpicture}
\end{document}

edited Nov 08 '21 at 06:27

answered Nov 07 '21 at 10:56

CarLaTeX

62,716

But I need to display labels only for those points that are in \points array – antshar Nov 07 '21 at 11:06
@antshar See my renewed answer, please. – CarLaTeX Nov 08 '21 at 06:28
That's an interesting workaround. If you fake ticks with drawings inside axis I don't see a reason to give up the initial array. I still don't know a way of converting this one into mine that can be iterated with an access to a specific index. So \foreach \i [evaluate={\ix=\points[\i][0]; \iy=\points[\i][1];}] in {0,...,\len}{ \addplot [only marks, opacity=0] coordinates {(\ix,0) (0,\iy)}; } will do. – antshar Nov 08 '21 at 15:24
@antshar Sorry, I don't understand what do you mean by "give up the initial array" – CarLaTeX Nov 08 '21 at 15:41
my array of points has {{{x1,y1},{x2,x2},...}} format, but yours is {(x1,y1),(x2,y2),...} – antshar Nov 08 '21 at 15:44
@antshar Ah, ok. It is because that is the syntax accepted by coordinates. If you can't change that, my solution is useless – CarLaTeX Nov 08 '21 at 15:48
your workaround with faking ticks is clever. It might be useful for someone else. Personally, I often come across with an interesting approach in posts that don't really relate to my search request, but still contain useful information. – antshar Nov 08 '21 at 15:53
@antshar Thank you! You're always very kind – CarLaTeX Nov 08 '21 at 15:54

muzimuzhi Z · Answer 2 · 2021-11-08T03:04:23.127

Here I use a \foreach to loop through \points and construct the list of x- and y-coords stored in \pointsX and \pointsY, respectively. Then just set xtick=\pointsX in axis environment.

Wrapping all these into a new pgfkeys handler would be neat, but the difference between a \foreach-list and a pgfmath-array makes the generalization very hard.

\documentclass{article}
\usepackage{etoolbox}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\def\points{{{-9,0.30},{-6,0.44},{-2,0.59},{3,1}}}
\pgfmathsetmacro{\len}{dim(\points)-1}
\def\pointsX{}
\def\pointsY{}
\foreach \i in {0,...,\len} {
  \pgfmathsetmacro{\x}{\points[\i][0]}
  \pgfmathsetmacro{\y}{\points[\i][1]}
  \ifnum\i=0
    \xappto\pointsX{\x}
    \xappto\pointsY{\y}
  \else
    \xappto\pointsX{,\x}
    \xappto\pointsY{,\y}
  \fi
}
\begin{axis}
[
  axis lines = center,
  axis line style = thick,
  xtick=\pointsX,
  ytick=\pointsY,
  clip=false
]
\addplot ({x},{x^2});
\end{axis}
\end{tikzpicture}
\end{document}

I see, etoolbox provides macro that appends elements. That's a neat solution that doesn't require much effort. Thank you for your answer. — antshar, Nov 08 '21 at 15:26
@antshar The dependence on etoolbox is soft. You can easily define your own \xappto by \def\myXappto#1#2{\xdef#1{\unexpanded\expandafter{#1}#2}} (this differs from the \xappto that it doesn't check the existance of #1). — muzimuzhi Z, Nov 08 '21 at 16:30

antshar · Accepted Answer · 2021-11-08T16:37:08.873

Basically the problem was about generating this kind of array {<num1>, <num2>, <num3>, ...}. It occurred to be quite tricky, but I managed to find the solution.

The key to solve the problem is an expansion. \xdef(equivalent to \global\edef) allows to define an expanded macro so that I it can access itself in its definition. For example

\edef\foo{-3}     % -> -3
\edef\foo{\foo,6} % -> -3,6

First solution based on \push macros that I found in this post. And the following code forms the desired array

\def\push#1#2{\expandafter\xdef\csname #1\endcsname{\expandafter\ifx\csname #1\endcsname\empty\else\csname #1\endcsname,\fi#2}}
\def\xarr{}
\def\yarr{}
\foreach \i [evaluate={\x=\points[\i][0]; \y=\points[\i][1]}] in {0,...,\len}{
    \push{xarr}{\x}
    \push{yarr}{\y}
}

Second solution doesn't require that macros, but still based on expanded definition

\def\xarr{}
\def\yarr{}
\foreach \i [evaluate={\x=\points[\i][0]; \y=\points[\i][1]; \c=ifthenelse(\i<\len,",",)}] in {0,...,\len}{
    \xdef\xarr{\xarr\x\c}
    \xdef\yarr{\yarr\y\c}
}

MWE

\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\def\points{{{-9,0.30},{-6,0.44},{-2,0.59},{3,1}}}
\pgfmathsetmacro{\len}{dim(\points)-1}
\def\push#1#2{\expandafter\xdef\csname #1\endcsname{\expandafter\ifx\csname #1\endcsname\empty\else\csname #1\endcsname,\fi#2}}
\def\xarr{}
\def\yarr{}
% First method
%\foreach \i [evaluate={\x=\points[\i][0]; \y=\points[\i][1]}] in {0,...,\len}{
%    \push{xarr}{\x}
%    \push{yarr}{\y}
%}
% Second method
\foreach \i [evaluate={\x=\points[\i][0]; \y=\points[\i][1]; \c=ifthenelse(\i<\len,",",)}] in {0,...,\len}{
    \xdef\xarr{\xarr\x\c}
    \xdef\yarr{\yarr\y\c}
}
\begin{axis}
[
axis lines = center,
axis line style = thick,
% xtick= loop here for \points[\i][0]
% ytick= loop here for \points[\i][1] where \i=0,...,\len
clip=false
]
\addplot ({x},{x^2});
\end{axis}
\end{tikzpicture}
\end{document}

Note that if you have babel package added, the second method might throw an error. That's all because of " inside ifthenelse. In order to fix that, use ifthenelse(\i<\len,\string",\string",), or use \shorthandoff{"}.

pgfplots add xtick from array

3 Answers3