\documentclass[border={10pt}]{standalone}
\usepackage{tikz,pgfplots,filecontents,amsmath}
\pgfplotsset{compat=1.5}

\begin{filecontents}{data.dat}
 n   yn 
-1   0
0    6
1    2
2    4
3    4
4    4
5    4
6    4
\end{filecontents}


\begin{document}
\begin{tikzpicture}
\begin{axis}
[%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    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}},
    xlabel={$k$},
    ylabel={$\boldsymbol{y[k]}$},
    xtick={0,1, ..., 6},
    xmax=7,
    ymin=0,
    ymax=8,
    yticklabels={
    	$0$,
    	$0$,
    	$0.5a+b$,
    	$1$,
    	$a$
    },
]%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\addplot+[ycomb,black,thick] table [x={n}, y={yn}] {data.dat};
%\addplot[mark=*] coordinates {(1,2)} node[pin=45:{}]{} ;
\addplot[] coordinates {(0,5.825)} node[pin=320:{arbitrary points at $x(0)$ and $x(1)$}]{} ;
\end{axis}
\end{tikzpicture}

\end{document}