使用 pgfplots 对绘图的部分进行阴影处理

Question

我有以下演示所需的图表。我有一个最小的代码，但是阴影是一个问题。我还没有完全掌握如何使用 PGFPlots。

\usepackage{tikz}
\usepackage{pgfplots}
\usepgfplotslibrary{fillbetween}
\usetikzlibrary{patterns}
\usetikzlibrary{decorations.markings}
\begin{figure}[h]
  \centering
  \begin{tikzpicture}[
    declare function={gamma(\z)=
(2.506628274631*sqrt(1/\z) + 0.20888568*(1/\z)^(1.5) + 0.00870357*(1/\z)^(2.5) - (174.2106599*(1/\z)^(3.5))/25920 - 
(715.6423511*(1/\z)^(4.5))/1244160)*exp((-ln(1/\z)-1)*\z);},
    declare function={gammapdf(\x,\k,\theta) = \x^(\k-1)*exp(-\x/\theta) / (\theta^\k*gamma(\k));}
]
    \begin{axis}[
    xmin=0, xmax=20,
    ymin=0, ymax=0.3,
    axis lines=middle,
  ]  
  \addplot[samples=100, domain=0:20, name path=A] {gammapdf(x,2,2)};
  \addplot[thick, samples=50, smooth,domain=0:6,magenta, name path=B ] coordinates {(7.4,0)(7.4,0.15)};
  \path[name path=xaxis] (\pgfkeysvalueof{/pgfplots/xmin}, 0) -- (\pgfkeysvalueof{/pgfplots/xmax},0);
  \addplot[gray, pattern=north west lines] fill between[of=A and xaxis, soft clip={domain=0:7.4}];
 \end{axis}
  \end{tikzpicture}
\end{figure} 

我有两个主要疑问：

1. 我不知道如何将交点标记为$Q^*$。
2. 我的阴影不起作用。
3. 另外我不知道外面的文字该怎么写。

Answer 1

1. 有几种命令可以实现这一点。我选择xtick在该位置设置一个，然后使用 来命名它xticklabels。
2. compat你说的一切都正确，但至少需要增加级别1.11。
3. 这很简单：只需在情节中的某处添加一个节点。

有关更多详细信息，请查看代码中的注释。

\documentclass[border=2mm]{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
    \usetikzlibrary{
        patterns,
        pgfplots.fillbetween,
    }
    \pgfplotsset{
        % by adding at least the `compat' level 1.11 the `fill between'
        % library works fine; also then you can omit `axis cs:' for the
        % node coordinates
        compat=1.11,
    }
\begin{document}
\begin{tikzpicture}[
    declare function={
        gamma(\z)=
            (2.506628274631*sqrt(1/\z) + 0.20888568*(1/\z)^(1.5)
                + 0.00870357*(1/\z)^(2.5) - (174.2106599*(1/\z)^(3.5))/25920
                - (715.6423511*(1/\z)^(4.5))/1244160)*exp((-ln(1/\z)-1)*\z);
        gammapdf(\x,\k,\theta)=
            \x^(\k-1)*exp(-\x/\theta) / (\theta^\k*gamma(\k));
    },
]
        % define value for Q*
        \def\Qstar{7.4}
    \begin{axis}[
        xmin=0, xmax=20,
        ymin=0, ymax=0.2,
        axis lines=middle,
        % add x axis label
        xlabel=$Q$,
        % change the position of `xlabel'
        xlabel style={
            anchor=west,
        },
        % add `xtick' only on the "Q*" position and name that tick appropriately
        xtick={\Qstar},
        xticklabels={$Q^*$},
        % remove `yticks'
        ytick=\empty,
    ]
        \addplot [samples=100,domain=0:20,name path=A] {gammapdf(x,2,2)};
        % draw a vertical path
        \path [name path=B]
            (\Qstar,\pgfkeysvalueof{/pgfplots/ymin}) --
            (\Qstar,\pgfkeysvalueof{/pgfplots/ymax});
        \path [name path=xaxis]
            (\pgfkeysvalueof{/pgfplots/xmin},0) --
            (\pgfkeysvalueof{/pgfplots/xmax},0)
        ;
        \addplot [gray,pattern=north west lines]
            fill between [of=A and xaxis, soft clip={domain=0:\Qstar}];
        % draw the vertical line up to the intersection with the function
        \draw [thick,magenta,intersection segments={of=A and B,sequence=R1}];

        % add the textbox
        \node [anchor=north east,align=center] at (axis description cs:1,1)
            {shaded area = 0.7 \\ = critical ratio};
    \end{axis}
\end{tikzpicture}
\end{document}