在我的 tikz 图像中，Fillbetween 不起作用

Question

我希望两个函数之间的空间有颜色，x 中的域为 0 到 4。但空间填充不起作用。这是我的代码

\documentclass{minimal}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{pgfplots}
\usepackage{tikz}
\pgfplotsset{compat=newest}
   \usetikzlibrary{positioning,arrows.meta}
   \usetikzlibrary{decorations.markings,bending}
   \usepgfplotslibrary{fillbetween}

\begin{document}
 
\begin{tikzpicture}
    \begin{axis}[
        scale = 0.8,
        xmin = 0, xmax = 5,
        ymin = 0, ymax = 50,
        axis lines = left,
        xtick = {0,4}, ytick = {48},
        clip = false,
        ]
    \addplot[domain = 0:5, restrict y to domain = 0:50, samples =
        400, color = red, name path = upaxis, postaction = {decorate}, decoration = {
                markings,
                   mark = at position 0.4 with {\arrow[black,line width=1pt,xscale=-1 {stealth}}
                  }]{12*(x)};                                                                       
    \addplot[domain = 0:5, restrict y to domain = 0:50, samples =
        400, color = black, name path = downaxis, postaction = {decorate}, decoration = {
                markings,
                   mark = at position 0.6 with {\arrow[line width=1pt]{stealth}}
                  }]{(3*(x)^2)};
    \addplot [blue, draw = none, fill opacity = 0.1] fill between [of
        = upaxis and downaxis, soft clip={domain=0:4}];
    \addplot[color = gray, dashed, ultra thin] coordinates {(4,0) (4,
        48) (0, 48)};
    \addplot[color = black, mark = *, only marks, mark size = 0.6pt]
        coordinates {(4, 48)};
    \node [right] at (current axis.right of origin) {$x$};
    \node [above] at (current axis.above origin) {$y$};
    \node [above left, draw, align = left] at (1.8, 31) {
        \textcolor{red}{$y=12x$} \\
        $y=3x^2$
    };
    \end{axis}
\end{tikzpicture}

\end{document}

我的代码结果是这样的

我想要的是这样的

Answer 1

\documentclass{minimal}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{pgfplots}
\usepackage{tikz}
\pgfplotsset{compat=newest}
\usetikzlibrary{positioning,arrows.meta}
\usetikzlibrary{decorations.markings,bending}
\usepgfplotslibrary{fillbetween}

\begin{document}
    
    \begin{tikzpicture}
        \begin{axis}[
            scale = 0.8,
            xmin = 0, xmax = 5,
            ymin = 0, ymax = 50,
            axis lines = left,
            xtick = {0,4}, ytick = {48},
            clip = false,
            ]
            \addplot[domain = 0:5, restrict y to domain = 0:50, samples =
            400, color = red,
            postaction = {name path = upaxis,},
            postaction = {decorate}, decoration = {
                markings,
                mark = at position 0.4 with {\arrow[black,line width=1pt,xscale=-1]{stealth}}
            }]{12*(x)};
            \addplot[domain = 0:5, restrict y to domain = 0:50, samples =
            400, color = black,
            postaction = {name path = downaxis,},
            postaction = {decorate}, decoration = {
                markings,
                mark = at position 0.6 with {\arrow[line width=1pt]{stealth}}
            }]{(3*(x)^2)};
            \addplot [blue, draw = none, fill opacity = 0.1] fill between [of
            = upaxis and downaxis, soft clip={domain=0:4}];
            \addplot[color = gray, dashed, ultra thin] coordinates {(4,0) (4,
                48) (0, 48)};
            \addplot[color = black, mark = *, only marks, mark size = 0.6pt]
            coordinates {(4, 48)};
            \node [right] at (current axis.right of origin) {$x$};
            \node [above] at (current axis.above origin) {$y$};
            \node [above left, draw, align = left] at (1.8, 31) {
                \textcolor{red}{$y=12x$} \\
                $y=3x^2$
            };
        \end{axis}
    \end{tikzpicture}
    
\end{document}

Answer 2

请尝试以下操作：

\documentclass[border=3.14159]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\usetikzlibrary{arrows.meta, 
                decorations.markings,
                intersections}
\usepgfplotslibrary{fillbetween}

\begin{document}
    \begin{tikzpicture}[
-|-/.style = {preaction=decorate, 
              decoration={markings,
                          mark=at position .45 with {#1{Straight Barb}}}
              },
dot/.style = {circle, fill=black, inner sep=1.2pt}
                        ]
\begin{axis}[
    axis lines = center,
    xlabel=$x$, 
    xtick = {0,4}, 
    ylabel=$y$,
    ytick = {48},
    label style = {anchor=north east},
    enlargelimits = {0.1,upper},
%
    legend style = {legend pos = north west, yshift=-3em},
%
    restrict y to domain=0:52,
    domain = 0:5, samples=51, no marks,
    every axis plot post/.append style={line width=1pt},
    clip=false
            ]
\addplot +[name path=A, -|- = \arrow]{12*(x)};
\addplot +[name path=B, -|- = \arrowreversed]{(3*(x)^2)};
\addplot [blue, fill opacity = 0.1]
    fill between [of = A and B];

\coordinate (origin) at (0,0);
\draw   [name intersections={of=A and B, by={x1,x2}}, 
         very thin, densely dashed]
        node[dot] at (x2) {}
        (origin |- x2) -| (origin -| x2);

\legend{$y=12x$, $y=3x^2$};
    \end{axis}
\end{tikzpicture}

\end{document}

编辑： 可以看到，装饰路径绘制为 \verb+preaction=decorate,+：

Answer 3

没有箭头也可以正常工作decoration- 会尝试找出原因吗？

\documentclass{minimal}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{pgfplots}
\usepackage{tikz}
\pgfplotsset{compat=newest}
   \usetikzlibrary{positioning,arrows.meta}
   \usetikzlibrary{decorations.markings,bending}
   \usepgfplotslibrary{fillbetween}

\begin{document}
 
\begin{tikzpicture}
    \begin{axis}[
        scale = 0.8,
        xmin = 0, xmax = 5,
        ymin = 0, ymax = 50,
        axis lines = left,
        xtick = {0,4}, ytick = {48},
        clip = false,
        ]
    \addplot[domain = 0:5, 
            restrict y to domain = 0:50, 
            samples = 400, 
            color = red, 
            name path = upaxis, 
            % postaction = {decorate}, 
            % decoration = {
            %     markings,
                   % mark = at position 0.4 with {\arrow[black,line width=1pt,xscale=-1 {stealth}}
                  % }
                  ]{12*(x)};                                                                       
    \addplot[domain = 0:5, 
            restrict y to domain = 0:50, 
            samples = 400, 
            color = black, 
            name path = downaxis, 
            % postaction = {decorate}, decoration = {
            %     markings,
                   % mark = at position 0.6 with {\arrow[line width=1pt]{stealth}}
                  % }
                  ]{(3*(x)^2)};
    \addplot [blue,] fill between [
                of = upaxis and downaxis, 
                soft clip={domain=0:4}
                ];
    \addplot[color = gray, dashed, ultra thin] coordinates {(4,0) (4,
        48) (0, 48)};
    \addplot[color = black, mark = *, only marks, mark size = 0.6pt]
        coordinates {(4, 48)};
    \node [right] at (current axis.right of origin) {$x$};
    \node [above] at (current axis.above origin) {$y$};
    \node [above left, draw, align = left] at (1.8, 31) {
        \textcolor{red}{$y=12x$} \\
        $y=3x^2$
    };
    \end{axis}
\end{tikzpicture}

\end{document}