pgfplots: setlength inside foreach

Question

I have the following code, where the size of the cube marks is set to \x inside a foreach loop:

\begin{tikzpicture}
 \begin{axis}[axis lines=none]
  \newlength{\mylen}
  \foreach \x in {1,2,...,10} {
  \setlength{\mylen}{\x pt}
    \addplot3[only marks,scatter,mark=cube*,mark size=\mylen] coordinates {(\x*5,10*\x,3*\x)};
    };
\end{axis}
\end{tikzpicture}

This results in an empty plot - i.e. it doesn't work.

However, if we set the length for mylen (to a constant) before the foreach-loop, it works just fine. That might be a clue. Any ideas?

Also, how can can I manipulate the length inside the \setlength command to for instance \setlength{\mylen}{2*\x+10 pt} ?

Answer 1

You should use the \pgfplotsinvokeforeach{list}{command} command for this, instead of the usual \foreach. That makes sure the parameter is passed to the \addplot options expanded, so when the plots are actually drawn (which does not happen immediately, I believe), they include the correct parameter:

\documentclass{article}
\usepackage{pgfplots}

\begin{document}

\begin{tikzpicture}
 \begin{axis}[axis lines=none]
  \pgfplotsinvokeforeach{1,2,...,10} {
    \addplot3[only marks,scatter,mark=cube*,mark size=#1 pt] coordinates {(#1*5,10*#1,3*#1)};
    };
\end{axis}
\end{tikzpicture}

\end{document}

Answer 2

The problem seems to be that the \addplot command doesn't interpret its options when it is encountered but only afterwards when the plot is drawn. This means that the current value of \x or \mylen inside the loop is not used but the latest value before or after it. The loop content is executed inside a group, so that changes don't survive afterwards.

In your case all plots where drawn using mark size=\mylen with \mylen having the default value of 0pt. The solution is to expand the value manually beforehand and call \addplot with the expanded value. You actually don't need the length for this. If you want to use it exchange \x with \the\mylen in the \edef part:

\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
 \begin{axis}[axis lines=none]
  \foreach \x in {1,2,...,10} {
        \edef\PLOT{\noexpand\addplot3[only marks,scatter,mark=cube*,mark size=\x pt]}
        \PLOT coordinates {(\x*5,10*\x,3*\x)};
    };
 \end{axis}
\end{tikzpicture}
\end{document}

Next time PLEASE provide a full compilable, but minimal example. I never used pgfplots before and had to guess the preamble. Here it turned out to be pretty simple but it can be quite complicated with PGF/TikZ when many libraries are required.