程序包 PGF 数学错误：未知运算符“x”或“x@”（在“x^2-3x”中）

Question

\documentclass{extarticle}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
  axis x line=center,
  axis y line=center,
  xtick=\empty,
  ytick=\empty,
  xlabel={$x$},
  ylabel={$y$},
  xlabel style={below right},
  ylabel style={above left},
  xmin=-5.5,
  xmax=5.5,
  ymin=-5.5,
  ymax=5.5]
\addplot [mark=none,domain=-4:4] {x^2-3x} node {$y=x^2-3x$};
\end{axis}
\end{tikzpicture}
\end{document}

错误消息是：包 PGF 数学错误：未知运算符x' orx@'（在'x^2-3x'中）。

Answer 1

简单。看一下我的建议：

\documentclass{extarticle}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
  axis x line=center,
  axis y line=center,
  xtick=\empty,
  ytick=\empty,
  xlabel={$x$},
  ylabel={$y$},
  xlabel style={below right},
  ylabel style={above left},
  xmin=-5.5,
  xmax=5.5,
  ymin=-5.5,
  ymax=5.5]
\addplot [domain=-4:5] {x^2 - 3*x};
\end{axis}
\end{tikzpicture}
\end{document}