[Tex/LaTex] Shading area under curve TikZ

pgfplotstikz-pgf

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}

Best Answer

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}

enter image description here

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.

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