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如何在 pgfplots 中使用交点坐标绘图？

tag-icon • tag-icon pgfplots coordinates intersections
如何在 pgfplots 中使用交点坐标绘图？

当使用 pgfplots 时，该命令\pgfgetlastxy无需轴环境即可正常工作，并且轴环境设置为默认环境。

但是在轴环境中设置某些东西时很难获得正确的相交坐标。

在此处输入图片描述

我想使用下面的 MWE 获得结果。

\documentclass{standalone}

%\usepackage{amsthm,amssymb}
\usepackage{pgfplots}
\usetikzlibrary{arrows,intersections,calc}

\begin{document}
\begin{tikzpicture}[font=\footnotesize]
\begin{axis}[%
axis lines*=none,axis y line=center, axis x line=center,
xlabel=$x$,ylabel=$y$,
every axis x label/.style={at={(current axis.right of origin)},anchor=north east,xshift=1mm},
every axis y label/.style={at={(current axis.above origin)},anchor=north east,yshift=1mm},
xmin=-0.2,ymin=-0.15,
xmax=1.5,ymax=1.5,
%xtick={1},ytick={1},
restrict y to domain=0:1.2,
height=6cm,width=5cm,
enlargelimits=false]
\node[below left,black] at (axis cs:0,0) {$O$};

\addplot[name path global=x2,blue,smooth,samples=500,domain=0:1.2] %
        {x^2} node[pos=0.75,sloped,yshift=-6pt]{$y=x^2$};
\addplot[name path global=genx,red,smooth,samples=500,domain=0:1.2] %
        {sqrt(x)} node[pos=0.75,sloped,yshift=6pt]{$x=y^2$};

\def\vlvalue{0.4}; %x  interval:[x,x+dx]
\def\intvlen{0.1}; %dx
\path[draw=none,name path=lx] (axis cs:\vlvalue,0)--(axis cs:\vlvalue,1.2); %x
\path[draw=none,name path=ldx] (axis cs:\vlvalue+\intvlen,0)--(axis cs:\vlvalue+\intvlen,1.2);%x+dx
%
\path[name intersections={of=x2 and lx,name=k}] (axis cs:\vlvalue,0)--(k-1); 
\path(k-1)  \pgfextra{\pgfgetlastxy{\kx}{\ky}     %%no ;
              \global\let\kx\kx %
              \global\let\ky\ky};
\draw[blue,dashed] (axis cs:\vlvalue,0)--(\kx,\ky);
\path[name intersections={of=genx and lx,name=m}] (axis cs:\vlvalue,0)--(m-1);
\path(m-1)  \pgfextra{\pgfgetlastxy{\mx}{\my}     %%no ;
              \global\let\mx\mx %
              \global\let\my\my};
%draw from (k-1) to (\vlvalue+\intvlen,\ky)
\draw[red] (\kx,\ky)--(axis cs:\vlvalue+\intvlen,\ky); % % %wrong \ky
%draw rectangle 
\fill[cyan!5] (k-1) rectangle (axis cs:\vlvalue+\intvlen,\my); % %wrong \my
\end{axis}
\end{tikzpicture}
\end{document}

答案1

您不需\pgfgetlastxy要这样做，您可以直接使用交叉点和calc库：

\documentclass{standalone}

%\usepackage{amsthm,amssymb}
\usepackage{pgfplots}
\usetikzlibrary{arrows,intersections,calc}

\begin{document}
\begin{tikzpicture}[font=\footnotesize]
\begin{axis}[%
axis lines*=none,axis y line=center, axis x line=center,
xlabel=$x$,ylabel=$y$,
xmin=-0.2,ymin=-0.15,
xmax=1.5,ymax=1.5,
restrict y to domain=0:1.2,
height=6cm,width=5cm,
enlargelimits=false]
\node[below left,black] at (axis cs:0,0) {$O$};

\addplot[name path global=x2,blue,smooth,samples=500,domain=0:1.2] %
        {x^2} node[pos=0.75,sloped,yshift=-6pt]{$y=x^2$};
\addplot[name path global=genx,red,smooth,samples=500,domain=0:1.2] %
        {sqrt(x)} node[pos=0.75,sloped,yshift=6pt]{$x=y^2$};

\def\vlvalue{0.4}; %x  interval:[x,x+dx]
\def\intvlen{0.1}; %dx
\path[name path=lx] (axis cs:\vlvalue,0)--(axis cs:\vlvalue,1.2);

\draw [
    red,
    name intersections={of=x2 and lx,name=k},
    name intersections={of=genx and lx,name=m}
    ] (k-1) rectangle ($(m-1)+(axis direction cs:\intvlen,0)$); % %wrong \my
\end{axis}
\end{tikzpicture}
\end{document}

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