Plot discrete graph with omitted/missing values

Question

I want to plot graph like there (plotting discrete signals) but with dots in values, I mean, dotted values on x without values of y, like "no information" (...). I slightly changed code from this thread but struggling with adding "dots" Below I attach example of what I want and my tex code with its output. I've been searching 1 hour over google, but without results.

Tex code

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usepackage{pgfplots}
\title{}
\author{}
\date{}
\begin{document}
\begin{filecontents}{data.dat}
 n   xn 
 0    10

 1    9

 2    5

 3    2

 4    -3
 5 -
 20    8
 21 3
\end{filecontents}
\begin{tikzpicture}
\begin{axis}
[%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    scale=1.3,
    axis x line=middle,
    axis y line=middle,
    every axis x label={at={(current axis.right of origin)},anchor=north west},
    every axis y label={at={(current axis.above origin)},anchor= north west},
    every axis plot post/.style={mark options={fill=black}},

    xmin=0,
    xmax=20, 
    xtick={0,1, 2, 3,4,5,20, 21},

    xticklabels={0,1, 2, 3, 4, .., 20, 21},
    xlabel={$\boldsymbol{x}$},
    ylabel={$\boldsymbol{z[n]}$},
    ytick={-5, 10},

    ymin=-5,
    ymax=10,
]%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\addplot+[ycomb,black, very thick] table [x={n}, y={xn}] {data.dat};
\end{axis}
\end{tikzpicture}
\end{document}

This is what I want to plot.

This is tex output.

This is kinda my drawing on output, describing what I want to plot.

Thanks for reading.

Answer 1

How about adding

\path[draw,loosely dotted,thick,black](6,-1)--(19,-1);

inside the axis environment?

Answer 2

How your problem can be solve, is shown in @ukg answer (+1), so here are some of topic suggestion haw can be improved )according to my opinion) your MWE:

\RequirePackage{filecontents}
    \begin{filecontents}{data.dat}
 n    xn
 0    10
 1     9
 2     5
 3     2
 4    -3
12    -3
13     2
14     5
20     8
21     3
25   nan
    \end{filecontents}
\documentclass[margin=3mm]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}
    \begin{tikzpicture}
\begin{axis}[x=4mm,
    axis lines=middle,
    axis on top,
    xlabel=$x$,

    ylabel=${z[n]}$,

    label style={anchor=north east},
    xtick=data,
    tick label style = {fill=white, inner sep=2pt, font=\scriptsize}, 
    extra x ticks = {0},
    ymin=-5, ymax=10,
    enlarge x limits={0.1},
    enlarge y limits={0.2,upper},
    every axis plot post/.style={very thick},
            ]
\addplot [ycomb, mark=*] table [x=n, y=xn] {zdata.dat};
\draw[very thick, dash pattern=on 1pt off 3pt, 
      transform canvas={yshift=-1.5ex}
      ]

        (6,0) -- (10,0)     % 6: position of 6th tick, 
                            % 10: position od 10th tick
        (16,0) -- (18,0);
\end{axis}
    \end{tikzpicture}
\end{document}