如何绘制垂直虚线？

Question

我想画一条垂直虚线 (1,0) – (1,3) 但是不起作用..

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows}
\usepackage{pgfplots} 

\usepackage[utf8]{inputenc}
\pgfplotsset{width=7cm,compat=1.9} 

\begin{document}

\begin{tikzpicture}
\begin{axis}[
  axis x line=center,
  axis y line=center,
   xlabel = {$\alpha$},
    ylabel = {$h(\alpha)$},
  xlabel style={ right},
  ylabel style={above },
]

\addplot [
    domain=-1.5:1.5, 
    samples=1000, 
    color=blue,
    ]
    {1*x + 1*x^2};

\end{axis}

\draw[dashed] (1,0) -- (1,3);

\end{tikzpicture}

\end{document}

Answer 1

使用最近的 LaTeX 安装和最新版本pgfplots包（你的版本很旧，是时候升级它了）：

\documentclass{article}
\usepackage{pgfplots} % it loads tikz too
\usetikzlibrary{arrows}
\pgfplotsset{width=7cm,compat=1.17} % <----

\begin{document}
    \begin{tikzpicture}
\begin{axis}[
  axis lines=center,
   xlabel = {$\alpha$},
   ylabel = {$h(\alpha)$},
  xlabel style={ right},
  ylabel style={above },
            ]
\addplot +[domain=-1.5:1.5, samples=100, thick, no marks]
    {1*x + 1*x^2};
\draw[dashed] (1,0) -- (1,3);  % <---
\end{axis}
    \end{tikzpicture}
\end{document}

Answer 2

将其放入环境中axis：

\draw [dotted,->] (axis cs:1,0) -- (axis cs:1,3);