TikZ/Pgfplots-将 expr 限制到域不接受 pgf 宏

Question

restrict expr to domain这是第一个问题，非常感谢大家的欢迎。当我尝试中的选项时\addplot，TikZ/PGF 似乎无法理解 生成的先前宏\pgfplotsextra，而它可以完美地与 生成的宏配合使用\def。

这是MWE：

\documentclass{standalone}
\usepackage{graphicx}
\usepackage{mathptmx}
\usepackage{tikz}
\usepackage{pgfplots}

% libraries
\usepgfplotslibrary{fillbetween}
\usetikzlibrary{intersections,calc, patterns}

% pgfplots settings
\pgfplotsset{compat=1.15}
\pgfplotsset{every axis/.append style={font=\small}}
\pgfplotsset{mystyle/.style={%
    clip=true,
    axis x line=bottom,
    axis y line=left,
    xmin = 0, xmax=21,
    ymax = 10, ymin  = 0}
}

\begin{document}

% Custom functions
\makeatletter
\def\markxof#1{
    \pgf@process{#1}
    \pgfmathparse{\pgf@x/\pgfplotsunitxlength+\pgfplots@data@scale@trafo@SHIFT@x)/10^\pgfplots@data@scale@trafo@EXPONENT@x}
}
\def\intmarkxof#1{
    \pgf@process{#1}
    \pgfmathparse{int(int(\pgf@x/\pgfplotsunitxlength+\pgfplots@data@scale@trafo@SHIFT@x)/10))}
}
\def\intmarkxofnext#1{
    \pgf@process{#1}
    \pgfmathparse{int(int(\pgf@x/\pgfplotsunitxlength+\pgfplots@data@scale@trafo@SHIFT@x)/10))+1}
}
\makeatother

% Initial values
\def\myrzero{8.5}
\def\myrdelta{-0.2}

\begin{tikzpicture}
    \begin{axis}[
        mystyle,
        grid = none,
        grid style={dashed},
        xlabel={\emph{$t_i$}},
        xlabel style={at={(1.05,-0.15)}, anchor=south},
        legend cell align=left,
        legend pos=outer north east
    ]
        % Revenue curve
        \addplot[name path global=revenue,blue,domain=0:20] {\myrzero * exp(\myrdelta * \x)};

        % Costs
        \addplot+[name path global=cost, ybar interval,mark=no, red, fill=white] plot coordinates { 
            (0, 0) (1, 0) (2, 0)  (3, 0.5) (4, 0.55) (5, 0.60) (6, 0.635) (7, 0.70) 
            (8, 0.725) (9, 0.75) (10, 0.8) (11, 0.85) (12, 0.95) (13, 1.15) (14, 1.25) 
            (15, 1.35) (16, 1.5) (17, 1.75) (18, 1.95) (19, 2) (20, 2.1)};

        % Intersection between revenue line and cost line
        \path [name intersections={of=revenue and cost, name=i}];
        \pgfplotsextra{
            \path (i-1) \pgfextra{\intmarkxof{i-1}\xdef\mylambda{\pgfmathresult}};
            \path (i-1) \pgfextra{\intmarkxofnext{i-1}\xdef\mylambdanext{\pgfmathresult}};
        }

        % Legend
        \legend{$r\left(t\right)$,$\left\{c_t\right\}$}

   \end{axis}
    % macros \mylambda and \mylambdanext are now defined

    \begin{axis}[
        mystyle,
        xtick=\empty, ytick=\empty,
        % Custom X ticks
        every extra x tick/.style={  
            tick0/.initial=1,
            tick1/.initial=7,
            xticklabel style={
                xshift=\pgfkeysvalueof{/pgfplots/tick\ticknum},
            },
        },
        extra x ticks={\mylambda,\mylambdanext},
        extra x tick labels={$t_\lambda$,$t_{\lambda+1}$},
        xticklabel style={anchor=north},
        % Custom Y ticks
        extra y ticks={\myrzero},
        extra y tick labels={$r_0$},
        extra y tick style={tickwidth=0mm},
        yticklabel style={xshift=17.5},
    ]

    % Highlight the cost bar at optimum point
    \addplot [name path global=cost2, ybar interval, mark=no, red, fill=red!50, restrict expr to domain={x}{\mylambda:\mylambdanext}] coordinates { 
        (0, 0) (1, 0) (2, 0)  (3, 0.5) (4, 0.55) (5, 0.60) (6, 0.635) (7, 0.70) 
        (8, 0.725) (9, 0.75) (10, 0.8) (11, 0.85) (12, 0.95) (13, 1.15) (14, 1.25) 
        (15, 1.35) (16, 1.5) (17, 1.75) (18, 1.95) (19, 2) (20, 2.1)};  

    % Draw optimum line to axis
    \draw[dashed] (i-1) -- ($(axis cs:0,-0.2)!(i-1)!(axis cs:20,-.2)$); 

    \end{axis}


\end{tikzpicture} 

\end{document}

更新（2）：谢谢成本加运费提供正确的解决方案。确实需要关闭当前轴并打开一个新轴才能访问\mylambda和\mylambdanext，正如 pgfplots 手册所指出的那样。我现在可以成功创建下图。