How do I draw a dashed line that overlays two adjacent tikz plots?

Question

I have tikz code that creates two plots, one above the other, using the scope environment with a shift and two separate axes. The plots look like this:

This is the code that creates them:

\documentclass[12pt,a4paper]{article}

\usepackage{float}
\usepackage[margin=1cm]{geometry}
\usepackage{pgfplots}
\pgfplotsset{compat=1.13}  

\begin{document}
\def\maxX{10}
\def\maxY{5}

\begin{figure}[H]
\begin{tikzpicture}

\begin{scope}
\begin{axis}[
    width=20cm, height=10cm,
    title={Top plot}, xlabel={}, ylabel={}, xticklabels={,,}, yticklabels={,,}, axis lines=middle,
    domain=0:10,
    samples=100,
    xmin=0, xmax=\maxX,
    ymin=0, ymax=\maxY
]
\addplot[black, thick] {1.8 - 0.2*x};
\addplot[black, thick] {3.0 - 0.4*x};
\addplot[black, thick] {2.8 - 0.6*x};
\addplot[black, thick] {3.8 - 0.6*x};
\addplot[black, thick] {3.8 - 0.8*x};
\addplot[black, thick] {5.0 - 1.2*x};
\end{axis}
\end{scope}

\begin{scope}[yshift=-10cm]
\begin{axis}[
    width=20cm, height=10cm,
    title={Bottom plot}, xlabel={}, ylabel={}, xticklabels={,,}, yticklabels={,,}, axis lines=middle,
    domain=0:10,
    samples=200,
    xmin=0, xmax=\maxX,
    ymin=0, ymax=\maxY
]
\addplot[red, very thick] { 2 * sin(deg(0.5*x + 6)) + 2.5 };
\end{axis}
\end{scope}

\end{tikzpicture}
\end{figure}
\end{document}

I'm trying to draw a dashed vertical line from the top of the top plot to the bottom of the bottom of the bottom plot, like this (made in Gimp):

How do I do this? I tried:

1. Adding a new axis with a vertical line drawn using coordinates, i.e. \draw[dashed] (axis cs:2,-10) -- (axis cs:2,10);, but this scales the plots according to that axis and messes up the spacing in the other two plots

2. Adding that same code, \draw[dashed] (axis cs:2,-10) -- (axis cs:2,10); to one of the current axes, but since they're clipped from 0 to 10, the dashed line doesn't extend the entire way

3. Outside of any axis, using a plain draw command: \draw[dashed] (2, -10) -- (2, 8.5); Since this is outside of an axis, it seems to use different coordinates, so the line is in the wrong horizontal position compared to the other two plots.

Answer 1

Add \coordinate (a) at (2,\pgfkeysvalueof{/pgfplots/ymax}); in the first axis environment and \coordinate (b) at (2,\pgfkeysvalueof{/pgfplots/ymin}); in the second. Then after the last \end{scope}, add \draw [dashed] (a) -- (b);.

As a sidenote, you might be interested in the groupplots library for aligned axes like this.

\documentclass[12pt,a4paper]{article}

\usepackage{float}
\usepackage[margin=1cm]{geometry}
\usepackage{pgfplots}
\pgfplotsset{compat=1.13}  
\usepgfplotslibrary{groupplots}
\begin{document}
% in general \newcommand over \def
\newcommand\maxX{10}
\newcommand\maxY{5}

\begin{figure}[H]
\begin{tikzpicture}
\begin{groupplot}[
    group style={
     group size=1 by 2,
     vertical sep=1.5cm
    },
    width=20cm, height=10cm,
    xlabel={}, ylabel={},
    xticklabels={,,}, yticklabels={,,},
    axis lines=middle,
    domain=0:10,
    xmin=0, xmax=\maxX,
    ymin=0, ymax=\maxY
]

\nextgroupplot[title={Top plot}, samples=2]
\addplot[black, thick] {1.8 - 0.2*x};
\addplot[black, thick] {3.0 - 0.4*x};
\addplot[black, thick] {2.8 - 0.6*x};
\addplot[black, thick] {3.8 - 0.6*x};
\addplot[black, thick] {3.8 - 0.8*x};
\addplot[black, thick] {5.0 - 1.2*x};
\coordinate (a) at (2,\pgfkeysvalueof{/pgfplots/ymax});

\nextgroupplot[title={bottom plot}, samples=100]
\addplot[red, very thick] { 2 * sin(deg(0.5*x + 6)) + 2.5 };
\coordinate (b) at (2,\pgfkeysvalueof{/pgfplots/ymin});
\end{groupplot}

\draw [dashed] (a) -- (b);
\end{tikzpicture}
\end{figure}
\end{document}