在轴环境中标记节点

Question

我不明白为什么以下代码不起作用。我收到错误：

./tabtest.tex:37: 未定义控制序列。
<argument> \xc
,\yc
l.37 \end{axis}

\documentclass{scrartcl}
%
\usepackage{pgfplotstable,tikz,filecontents}
%
\begin{document} 
    \begin{filecontents}{data.txt}
         0.84661730E-01  0.53453308E+00 -
         0.37594643E+00  0.38930386E+00 +
         0.54111367E+00  0.94450973E-02 +
         0.38930386E+00 -0.37594646E+00 -
         0.94450731E-02 -0.54111367E+00 -
        -0.37594652E+00 -0.38930380E+00 +
        -0.54111367E+00 -0.94451150E-02 +
        -0.38930374E+00  0.37594658E+00 -
    \end{filecontents}
%
    \begin{tikzpicture}
        \begin{axis}[axis equal, axis lines=none, xmin=-1, xmax=1, ymin=-1, ymax=1, clip=false]
            \coordinate (OP) at (0,0);
            \draw [black] (OP) circle (1);
            %
            \pgfplotstableread[header=false]{data.txt}{\datatable}
            \pgfplotstablegetrowsof{\datatable}
            \pgfmathsetmacro{\rownum}{\pgfplotsretval-1}
            %
            \addplot[mark=none] table {\datatable};
            %
            \foreach \angle in {0,...,\rownum}{%
                \pgfplotstablegetelem{\angle}{[index]0}\of{\datatable}
                \let\xc\pgfplotsretval
                \pgfplotstablegetelem{\angle}{[index]1}\of{\datatable}
                \let\yc\pgfplotsretval
                \pgfplotstablegetelem{\angle}{[index]2}\of{\datatable}
                \let\ps\pgfplotsretval
                \node at (\xc,\yc) {\ps};
            }%
        \end{axis}
    \end{tikzpicture}
\end{document}