pgfplots: how to check if addplot filters all data

Question

is there an easy way to check if a \addplot(+) table [...] command in PGFPLOTS has any drawn any points? The reason is that I want to process the data dynamically, and if the plot is empty, the legend should be omitted.

Here is a MWE:

\documentclass{article}

\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{pgfplotstable}

\pgfkeys{
        /tr/rowfilter/.style 2 args={
                /pgfplots/x filter/.append code={
                        \edef\arga{\thisrow{#1}}
                        \edef\argb{#2}
                        \ifx\arga\argb
                        \else
                                \def\pgfmathresult{}
                        \fi
                }
        }
}

\usepackage{filecontents}
\begin{filecontents}{diagram.dat}
x y kind
1 10 a
1 20 a
2 13 b
2 15 b
\end{filecontents}
\begin{document}

\begin{tikzpicture}
\begin{axis}[filter discard warning=false]
    \addplot table[/tr/rowfilter={kind}{a},/tr/rowfilter={x}{2}] {diagram.dat}; % if the plot doesn't contain curve a,
    \addlegendentry{curve a}                                                    % <- don't execute this line
    \addplot table[/tr/rowfilter={kind}{b},/tr/rowfilter={x}{2}] {diagram.dat};
    \addlegendentry{curve b}
\end{axis}
\end{tikzpicture}

\begin{tikzpicture}
\begin{axis}[filter discard warning=false]
    \addplot table[/tr/rowfilter={kind}{a},/tr/rowfilter={x}{1}] {diagram.dat};
    \addlegendentry{curve a}
    \addplot table[/tr/rowfilter={kind}{b},/tr/rowfilter={x}{2}] {diagram.dat};
    \addlegendentry{curve b}
\end{axis}
\end{tikzpicture}

\end{document}

In the first diagram the legend is wrong: the line belongs to dataset of curve b! I thought of something like \newif\ifplotexists, but, as there are two filters, how to decide if \plotexistsfalse or \plotexiststrue should be set?

Answer 1

If you supply \addlegendentry using execute at end plot visualization=... in the \addplot options, the legend entry will only be added if the plot is not completely filtered away:

\documentclass{article}

\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{pgfplotstable}

\pgfkeys{
        /tr/rowfilter/.style 2 args={
                /pgfplots/x filter/.append code={
                        \edef\arga{\thisrow{#1}}
                        \edef\argb{#2}
                        \ifx\arga\argb
                        \else
                                \def\pgfmathresult{}
                        \fi
                }
        }
}

\usepackage{filecontents}
\begin{filecontents}{diagram.dat}
x y kind
1 10 a
1 20 a
2 13 b
2 15 b
\end{filecontents}
\begin{document}

\begin{tikzpicture}
\begin{axis}[filter discard warning=false]
    \addplot +[execute at end plot visualization=\addlegendentry{A}] table[/tr/rowfilter={kind}{a},/tr/rowfilter={x}{2}] {diagram.dat}; % if the plot doesn't contain curve a,                                               % <- don't execute this line
    \addplot +[execute at end plot visualization=\addlegendentry{B}] table[/tr/rowfilter={kind}{b},/tr/rowfilter={x}{2}] {diagram.dat};
\end{axis}
\end{tikzpicture}

\begin{tikzpicture}
\begin{axis}[filter discard warning=false]
    \addplot +[execute at end plot visualization=\addlegendentry{A}] table[/tr/rowfilter={kind}{a},/tr/rowfilter={x}{1}] {diagram.dat};
    \addplot  +[execute at end plot visualization=\addlegendentry{B}] table[/tr/rowfilter={kind}{b},/tr/rowfilter={x}{2}] {diagram.dat};
\end{axis}
\end{tikzpicture}

\end{document}