在图例中绘制实心圆或点

Question

我正在尝试创建带有图例中实心圆圈的图。请参见下图了解我目前得到的结果：

我无法正确绘制图例，理想情况下，我想绘制带有标签 Maxima 的实心红色圆圈和带有标签 Minima 的实心蓝色圆圈。但是，我不确定如何在图例中创建实心圆圈。

请参阅下面的代码：

\begin{tikzpicture}
  \begin{axis}[
    trig format plots=rad,
    axis lines = middle,
    enlargelimits,
    clip=false,
    legend entries={Minima,Maxima}
    ]
    \addplot[domain=-2*pi:2*pi,samples=200,black] {sin(x)};
    \fill[red] (axis cs: 0.5*pi,1.0) circle[radius=3pt];
    \fill[red] (axis cs: -1.5*pi,1.0) circle[radius=3pt];
    \fill[blue] (axis cs: -0.5*pi,-1.0) circle[radius=3pt];
    \fill[blue] (axis cs: 1.5*pi,-1.0) circle[radius=3pt];
    \draw[dotted,red!80] (axis cs: -1.5*pi,1.1) -- (axis cs: -1.5*pi,0);
    \draw[dotted,red!80] (axis cs: 0.5*pi,1.1) -- (axis cs: 0.5*pi,0);
    \draw[dotted,blue!80] (axis cs: 1.5*pi,-1.1) -- (axis cs: 1.5*pi,0.0);
    \draw[dotted,blue!80] (axis cs: -0.5*pi,-1.1) -- (axis cs: -0.5*pi,0.0);
    \addlegendimage{mark=ball,ball color=red,draw=red}
    \addlegendimage{mark=ball,ball color=blue,draw=blue}
  \end{axis}
\end{tikzpicture}

任何帮助或建议都将不胜感激。

Answer 1

对于这个简单的案例，你可以假装

\begin{tikzpicture}
\begin{axis}[
    trig format plots=rad,
    axis lines = middle,
    enlargelimits,
    clip=false,
    legend entries={Maxima,Minima}
    ]
    \addplot[domain=-2*pi:2*pi,samples=101, black,forget plot] {sin(x)};
    \addplot[domain=-3*pi/2:pi/2,samples=2,only marks,mark options=red] {sin(x)<0?NaN:sin(x)};
    \addplot[domain=-pi/2:3*pi/2,samples=2,only marks,mark options=blue] {sin(x)>0?NaN:sin(x)};
  \end{axis}
\end{tikzpicture}