绘图显示不正确

Question

我正在绘制一个函数图，我想在轴上标记两个点。其中一个点标记正确，但另一个点显示的是数字 4，而不是相应的字母。我该如何解决这个问题？

\documentclass{report}
\usepackage{pgfplots}

\begin{document}

\begin{tikzpicture}
  \begin{axis}[
    axis lines=middle,
    xlabel=$x$,
    ylabel=$f_X(x)$,
    ymin=0,
    ymax=0.6,
    xmax=8,
    samples=100,
    domain=-2:10,
    every axis x label/.style={at={(current axis.right of origin)},anchor=west},
    every axis y label/.style={at={(current axis.above origin)},anchor=south},
    yticklabels={},
    xticklabel={$\mu$},
    xtick={4},
  ]
    
    \pgfmathsetmacro{\mu}{4}
    \pgfmathsetmacro{\theta}{1}
    \addplot[blue, thick] {1/(sqrt(2*pi*\theta^2))*exp(-1/(2*\theta^2)*(x-\mu)^2)};
    \node[below] at (axis cs:\mu, 0) {$\sim$};
    \node[left] at (axis cs:0, {1/(sqrt(2*pi*\theta^2))}) {$\frac{1}{\sqrt{2\pi\theta^2}}$};
    \draw[dashed] (axis cs: \mu,0) -- (axis cs: \mu, {1/(sqrt(2*pi*\theta^2))});
  
  \end{axis}
\end{tikzpicture}

\end{document}