pgfplot 轴中的绘制命令不正确

Question

我希望绘制一个圆形的半径，如下图所示：

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.5.1}
\begin{document}
    \begin{tikzpicture}
    \begin{axis}[
    legend pos=outer north east,
    legend cell align={left},
    grid, grid style=dashed,
    xmin=0,xmax=1.2,
    ymin=0,ymax=1.2,
    ytick={0,0.2,...,1.2},
    xtick={0,0.2,...,1.2},
    extra y ticks={0.5},
    extra x ticks={0.5},
    axis lines = middle,
    set layers,
    xlabel={$x$},ylabel={$y$}, 
    x label style={at={(1,0)},right},
    y label style={at={(0,1)},above},
    x tick label style={
        tick label style={rotate=60},
        anchor=east
    },
    y tick label style={
        /pgf/number format/fixed,
        /pgf/number format/precision=1,
        /pgf/number format/fixed zerofill=true
    },
    axis equal,
    ]
    \node[fill,circle,minimum size=2pt,inner sep=0] at (axis cs:0.5,0.5) {};
    \draw[line width=1pt] (axis cs:0.5,0.5) circle (0.5);
    \draw (0.5,0.5) -- ++(0.5,0);
    \end{axis}
    \end{tikzpicture}
\end{document}

但该单位听起来不像预期的那样：

Answer 1

您使用的是旧版本的pgfplots。因此，您需要在坐标前加上axis cs:，就像您对圆所做的那样。相对坐标更加棘手，因为(axis cs:0,0)可能不在(0,0)环境 处tikzpicture，请参阅Torbjørn 的链接。但是，我想说的是，在这种情况下，使用相对坐标的优势最多只能说是微不足道。所以，假设你希望保留旧的兼容模式，

\draw (axis cs:0.5,0.5) -- (axis cs:1,0.5);

是最简单的方法。

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.5.1}
\begin{document}
    \begin{tikzpicture}
    \begin{axis}[
    legend pos=outer north east,
    legend cell align={left},
    grid, grid style=dashed,
    xmin=0,xmax=1.2,
    ymin=0,ymax=1.2,
    ytick={0,0.2,...,1.2},
    xtick={0,0.2,...,1.2},
    extra y ticks={0.5},
    extra x ticks={0.5},
    axis lines = middle,
    set layers,
    xlabel={$x$},ylabel={$y$}, 
    x label style={at={(1,0)},right},
    y label style={at={(0,1)},above},
    x tick label style={
        tick label style={rotate=60},
        anchor=east
    },
    y tick label style={
        /pgf/number format/fixed,
        /pgf/number format/precision=1,
        /pgf/number format/fixed zerofill=true
    },
    axis equal,
    ]
    \node[fill,circle,minimum size=2pt,inner sep=0] at (axis cs:0.5,0.5) {};
    \draw[line width=1pt] (axis cs:0.5,0.5) circle[radius=0.5];
    % or circle[x radius=\x1,y radius=\y1];
    \draw (axis cs:0.5,0.5) -- (axis cs:1,0.5) node[midway,above]{$r$};
    \end{axis}
    \end{tikzpicture}
\end{document}