Using addplot and pgfmathresult for color

Question

I could use some help. I am trying to color a bar chart assigning different colors using a foreach loop and pgfmathresult to get a color gradient. Here is a minimal example:

\documentclass{minimal}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\begin{document}
\pgfplotstableread{
Label 1 2 3
R1 3 2 1
R2 2 1 3
R3 1 2 3
}\testdata
\begin{tikzpicture}
\begin{axis}[ybar stacked, ymin=0, xtick=data, xticklabels from table={\testdata}{Label}]
\foreach \s in {1,...,3}{
    \pgfmathparse{\s*30}
    \addplot [fill=blue!\pgfmathresult!green] table [y=\s, meta=Label,x expr=\coordindex] {\testdata};
}
\end{axis}
\end{tikzpicture}
\end{document}

But the result I obtain has no gradient

:

Jet when I run almost the same code (without pgfplot):

\begin{tikzpicture}
\foreach \s in {1,...,3} {
    \pgfmathparse{(\s*30)}
 \draw[blue!\pgfmathresult!green, thick]
(0,\s * .2) -- (1,\s * .2)
;
}
\end{tikzpicture}

I get the desired gradient:

Has anyone an idea whats going wrong here and how to fix it?

I guess addplot changes the value of pgfmathresult while working with it. Try temporarily store the value to something else. — user202729, Apr 19 '22 at 11:10
tikz pgf - Store \pgfmathresult in a variable - TeX - LaTeX Stack Exchange — user202729, Apr 19 '22 at 11:37
This seems quite promising but I cant get it to work: https://tex.stackexchange.com/questions/59817/using-value-of-a-table-as-color-indicator-for-addplot-like-a-colormap?rq=1 — Florian, Apr 19 '22 at 11:43
Okay I figure out the solution, turns out the thing stores the content verbatim until the end of the block before expanding the content inside... — user202729, Apr 19 '22 at 11:46

user202729 · Accepted Answer · 2022-04-19T12:17:14.793

You need to expand pgfmathresult before passing it to \addplot. To inject the expanded value inside the command, one way is to use a common idiom LaTeX3 - Expand nested argument

(side note, addplot will eventually expand it, but only at the end; thus all the colors get expanded to the same value)

\def\plotcommand#1{
    \addplot [fill=blue!#1!green] table [y=\s, meta=Label,x expr=\coordindex] {\testdata};
}
\begin{tikzpicture}
\begin{axis}[ybar stacked, ymin=0, xtick=data, xticklabels from table={\testdata}{Label}]
\foreach \s in {1,...,3}{
    \pgfmathparse{\s*30}
    \expandafter\plotcommand\expandafter{\pgfmathresult}
}
\end{axis}
\end{tikzpicture}

Note

~~I'm not that familiar with TikZ, maybe TikZ offer a built-in solution that is easier to read.~~ On reading the questions below I'm pretty convinced that it isn't really possible to be cleaner.
With a new LaTeX kernel you can \ExpandArgs{V}\plotcommand\pgfmathresult, see How do I have to invoke \expandafter for a macro with multiple arguments?
If you put the plotcommand command definition inside the foreach you need to double the # because of some weird foreach command "bug" _{I can find Having trouble with Tikz and Beamer which is similar but not exactly the same}
If you don't mind using heavy package for a one-off task I'd recommend my package

\usepackage{execinside}
[... common preamble ...]
\begin{tikzpicture}
\begin{axis}[ybar stacked, ymin=0, xtick=data, xticklabels from table={\testdata}{Label}]
\foreach \s in {1,...,3}{
    \pgfmathparse{\s*30}
    \execinside{
        \addplot [fill=blue!\EIexpand\pgfmathresult!green] table [y=\s, meta=Label,x expr=\coordindex] {\testdata};
    }
}
\end{axis}
\end{tikzpicture}

Using addplot and pgfmathresult for color

1 Answers1