Shading area under curve TikZ

Question

Using code I got from here, I'm trying to make a small rectangle of height y = 6 between bounds x = 2 and x = 7, and shade it in.

Any idea why this doesn't work?

\documentclass[12pt]{standalone}
\usepackage{pgfplots}
\usepackage{tkz-euclide}
\usepgfplotslibrary{fillbetween}
\pgfplotsset{width=10cm,compat=1.9}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
  axis x line=middle, axis y line=middle,
  ymin=0, ymax=10, ytick={0,2,...,10}, ylabel=$f(x)$,
  xmin=0, xmax=10, xtick={0,2,...,10}, xlabel=$x$,
  domain=-pi:pi,samples=101, % added
]
    \addplot[domain=2:7,blue,name path=A] {6}; % actual curve
    \addplot[draw=none,name path=B] {0};     % “fictional” curve
    \addplot[gray] fill between[of=A and B,soft clip={domain=2:7}]; %filling
\addplot+[
      blue,very thick,dotted,
      mark=none,
      const plot,
      empty line=jump,
]
coordinates {
    (2,0)
    (2,6)

    (7,0)
    (7,6)
};
\end{axis}
\end{tikzpicture}

\end{document}

Answer 1

It does not work because the domain domain=-pi:pi, which you set in the axis options, is used for the plot named B. If you add the appropriate domain for that plot, it works as expected.

\documentclass[12pt]{standalone}
\usepackage{pgfplots}
\usepackage{tkz-euclide}
\usepgfplotslibrary{fillbetween}
\pgfplotsset{width=10cm,compat=1.16}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
  axis x line=middle, axis y line=middle,
  ymin=0, ymax=10, ytick={0,2,...,10}, ylabel=$f(x)$,
  xmin=0, xmax=10, xtick={0,2,...,10}, xlabel=$x$,
  domain=-pi:pi,samples=101, % added
]
    \addplot[domain=2:7,blue,name path=A] {6}; % actual curve
    \addplot[draw=none,name path=B,domain=2:7] {0}; 

\addplot[gray] fill between[of=A and B,soft clip={domain=2:7}]; %filling
\addplot+[
      blue,very thick,dotted,
      mark=none,
      const plot,
      empty line=jump,
]
coordinates {
    (2,0)
    (2,6)

    (7,0)
    (7,6)
};
\end{axis}
\end{tikzpicture}
\end{document}  

ADDENDUM: Just to show Stefan Pinnow that his proposal is certainly not the simplest one. This code

\documentclass[12pt]{standalone}
\usepackage{pgfplots}
\usepackage{tkz-euclide}
\usepgfplotslibrary{fillbetween}
\pgfplotsset{width=10cm,compat=1.16}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
  axis x line=middle, axis y line=middle,
  ymin=0, ymax=10, ytick={0,2,...,10}, ylabel=$f(x)$,
  xmin=0, xmax=10, xtick={0,2,...,10}, xlabel=$x$,
  domain=-pi:pi,samples=101, % added
]

    \fill[gray] (2,0) rectangle (7,6);

\addplot+[
      blue,very thick,dotted,
      mark=none,
      const plot,
      empty line=jump,
]
coordinates {
    (2,0)
    (2,6)

    (7,0)
    (7,6)
};
\end{axis}
\end{tikzpicture}
\end{document}  

yields the same output. But this is not the point of the discussion. The point, I think, is that your code yields an unexpected (or "funky" as they were called before the edit) results. To shade a rectangle, you do not need even pgfplots, actually not even TikZ.

Answer 2

As marmot already pointed out in his answer what causes the mess is that path B has the wrong domain. The simplest way to get what you want is to set the domain in the axis options (only) and remove it everywhere else.

But to simply draw a rectangle you don't need the fillbetween library. For details please have a look at the comments in the code.

% used PGFPlots v1.16
\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
    \usepgfplotslibrary{fillbetween}
    \pgfplotsset{width=10cm,compat=1.16}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        axis x line=middle,
        axis y line=middle,
        ymin=0,
        ymax=10,
        ytick={0,2,...,10},
        ylabel=$f(x)$,
        xmin=0,
        xmax=10,
        xtick={0,2,...,10},
        xlabel=$x$,
        domain=2:7, % <-- adapted to the "right" bounds
        samples=2,  % <-- (for straight lines 2 is enough)
    ]

        % alternative without using the `fillbetween' library
        \addplot [fill=red,opacity=0.5] coordinates { (2,6) (7,6) }
            \closedcycle
        ;

        \addplot [blue,name path=A] {6};
        \addplot [draw=none,name path=B] {0};
        \addplot [gray] fill between [
            of=A and B,
%            soft clip={domain=2:7}     % <-- not needed any more now
        ];

        \addplot+ [
              blue,very thick,dotted,
              mark=none,
              const plot,
              empty line=jump,
        ] coordinates {
            (2,0)
            (2,6)

            (7,0)
            (7,6)
        };

    \end{axis}
\end{tikzpicture}
\end{document}