Changing parameters in a groupplot

Question

The idea behind the following sample code is this: There should be four plots, and for each plot two parameters should be set. This is achieved through \def just before \nextgroupplot. Apparently, this approach is not sound.

\documentclass{beamer}
\setbeamertemplate{navigation symbols}{}

\usepackage{pgfplots}
\pgfplotsset{compat=1.7}
\usepgfplotslibrary{groupplots}

\begin{document}

\begin{frame}
  \begin{tikzpicture}
    \begin{groupplot}[group style={group size=2 by 2}, height=5cm, width=6cm]
      \def\myA{1}
      \def\myB{1}
      \nextgroupplot[xlabel = {A: \myA, B: \myB} ]
      \addplot coordinates{(0,0) (1,\myA) (2,\myB)};

      \def\myA{1}
      \def\myB{2}
      \nextgroupplot[xlabel = {A: \myA, B: \myB} ]
      \addplot coordinates{(0,0) (1,\myA) (2,\myB)};

      \def\myA{2}
      \def\myB{1}
      \nextgroupplot[xlabel = {A: \myA, B: \myB} ]
      \addplot coordinates{(0,0) (1,\myA) (2,\myB)};

      \def\myA{2}
      \def\myB{2}
      \nextgroupplot[xlabel = {A: \myA, B: \myB} ]
      \addplot coordinates{(0,0) (1,\myA) (2,\myB)};
    \end{groupplot}
  \end{tikzpicture}
\end{frame}
\end{document}

Here's the output of the above snippet: .

If everything worked as planned, we would see four different figures, with different values at x=1 and x=2, labelled like this:

A: 1, B:1   A: 1, B:2
A: 2, B:1   A: 2, B:2

Neither works. All four plots appear to take their parameters from the very first definition only. For the labels, the order is changed (2-3-4-1 rather than 1-2-3-4).

Q: Why is that? What's the proper way to do what I mean to do?

Q: Why can't I write that code as

\documentclass{beamer}
\setbeamertemplate{navigation symbols}{}

\usepackage{pgfplots}
\pgfplotsset{compat=1.7}
\usepgfplotslibrary{groupplots}

\begin{document}

\def\myAs{1,2}
\def\myBs{1,2}

\begin{frame}
  \begin{tikzpicture}
    \begin{groupplot}[group style={group size=2 by 2}, height=5cm, width=6cm]
      \foreach\myA in \myAs{
        \foreach\myB in \myBs{
          \nextgroupplot[xlabel = {A: \myA, B: \myB} ]
          \addplot coordinates{(0,0) (1,\myA) (2,\myB)};
        }
      }
    \end{groupplot}
  \end{tikzpicture}
\end{frame}
\end{document}

Answer 1

The pgfplots package doesn’t execute everything right away but does collect the plots first, calculated stuff and then actually draws something. With the etoolbox package you can use your two loops (at least in this example) pretty easily to build a macro that contains all Plots with expanded values \myA and \myB.

Code

\documentclass{beamer}
\setbeamertemplate{navigation symbols}{}

\usepackage{pgfplots,etoolbox}
\pgfplotsset{compat=1.7}
\usepgfplotslibrary{groupplots}

\begin{document}

\def\myAs{1,2}
\def\myBs{1,2}

\begin{frame}
  \begin{tikzpicture}
    \begin{groupplot}[group style={group size=2 by 2}, height=5cm, width=6cm]
      \def\myPlots{}
      \pgfplotsforeachungrouped \myA in {1,2}{
        \pgfplotsforeachungrouped \myB in {1,2}{
          \eappto\myPlots{%
            \noexpand\nextgroupplot[xlabel = {A: \myA, B: \myB} ]
            \noexpand\addplot coordinates{(0,0) (1,\myA) (2,\myB)};
          }
        }
      }
      \myPlots
    \end{groupplot}
  \end{tikzpicture}
\end{frame}
\end{document}