# Externalizing pgfplots/gnuplots in Beamer — contour gets computed for every slide

I am trying to externalize pgfplots, gnuplot and contour gnuplot in beamer. Towards this end I am using the visible on=<> facility of \usetikzlibrary{overlay-beamer-styles} (cf. this answer by user Daniel) as well as the automatic numbering solution from user Loop Space.

I created a MWE consisting of 3 frames each hosting 3 slides that contain (A) vanilla pgfplots (B) gnuplots (C) contour gnuplots.

Expected Behaviour: When externalizing, say the images of the second frame, images on other frames should be ignored.

Actual Behaviour: The contour plots get computed in every single overlay. In total 81 sets of contour plots get computed. If one deletes the first 2 frames (which do not contain contour plots), then still 27 sets of contours get computed.

How to fix this bug?

\documentclass{beamer}
\usepackage{tikz, pgfplots}
\pgfplotsset{compat=1.16}
\usepgfplotslibrary{external}
\usetikzlibrary{overlay-beamer-styles}

% automatic beamer numbering
%\url{https://tex.stackexchange.com/q/119428/86}
\makeatletter
\newcommand*{\overlaynumber}{\number\beamer@slideinframe}
\tikzset{
beamer externalizing/.style={%
execute at end picture={%
\tikzifexternalizing{%
\ifbeamer@anotherslide
\pgfexternalstorecommand{\string\global\string\beamer@anotherslidetrue}%
\fi
}{}%
}%
},
external/optimize=false
}
\let\orig@tikzsetnextfilename=\tikzsetnextfilename
\renewcommand\tikzsetnextfilename[1]{\orig@tikzsetnextfilename{#1-\overlaynumber}}
\makeatother

\tikzset{every picture/.style={beamer externalizing}}
\tikzexternalize[only named=true]

\begin{document}
\begin{frame}{pgfplots}
\tikzsetnextfilename{pgfplots}
\begin{tikzpicture}
\begin{axis}
\end{axis}
\end{tikzpicture}
\end{frame}

\begin{frame}{gnuplot}
\tikzsetnextfilename{gnuplot}
\begin{tikzpicture}
\begin{axis}
\end{axis}
\end{tikzpicture}
\end{frame}

\begin{frame}{gnuplot contour}
\tikzsetnextfilename{contour}
\begin{tikzpicture}
\begin{axis}[view={0}{90}, domain=-2:2]
\addplot3[visible on=<1-3>,contour gnuplot={draw color=red,number=20, labels=false}] {x};
\addplot3[visible on=<2-3>,contour gnuplot={draw color=blue,number=20, labels=false}] {y};
\addplot3[visible on=<3-3>,contour gnuplot={draw color=black,number=20, labels=false}] {x^2+y^2};
\end{axis}
\end{tikzpicture}
\end{frame}
\end{document}

• getting 9 contour on build – js bibra Jan 5 at 15:05
• @jsbibra I am compiling with pdflatex --shell-escape (version 3.14159265-2.6-1.40.20). After successful compilation there are a total of 312 files, 81 of the form contourmpX.table. i.e. gnuplot gets called internally 81 times instead of 9 times, which is a huge wast of resources – Hyperplane Jan 5 at 15:18
• @jsbibra The problem is not that the .pdf does not compile to what I want it to, but that gnuplot contour get called and computes the whole contour plot for every frame, even the ones the plot isn't even on. This makes compilation unfeasible for large documents (the actual contour plot I want to externalize is 9 slides and alone takes 2 mins to compile) – Hyperplane Jan 5 at 15:21