Add vertical lines from ymin to ymax

Question

I am trying to add vertical lines to my plot. The problem is that I don't know what ymax is, so I cannot just draw them easily. So far I have been able to:

But I don't know how to shift the top position in a similar way to the bottom.

I can move the top position manually as e.g:

\addplot[draw=gray] (16*\i-0.5,0) --                             
                ($ (current axis.above origin)+(15.5,0) $);

But it fails when I do it with the variable \i as:

\addplot[draw=gray] (16*\i-0.5,0) --                             
                ($ (current axis.above origin)+(16*\i-0.5,0) $);

I get the error:

! Undefined control sequence.
\pgfmath@dimen@ ...men@@ #1=0.0pt\relax \pgfmath@
l.38            \end{axis}
? 
)
Runaway argument?
\def \expandafter \pgfmath@number \expandafter {\the \pgfmath@count }\ETC.
! File ended while scanning use of \pgfmath@dimen@@.
<inserted text> 
                \par 
<*>

Furthermore, the lines don't go all the way to the top, they only go up until the ticks. How can I make the lines go all the way to the top (or move the ticks to match the lines only in the top)?

\documentclass[11pt]{article}                                                    
\usepackage[top=2.54cm,bottom=2.54cm,left=2.75cm,right=2.75cm]{geometry}         
\usepackage{tikz}                                                                
\usetikzlibrary{calc}                                                            
\usepackage{pgfplots}
\begin{filecontents}{data.txt}

0 10000

1 15000

2 10000

3 20000

4 5000

5 0

\end{filecontents}

\begin{document}

\begin{figure}

    \centering

    \begin{tikzpicture}

        \begin{axis} [

                height = \axisdefaultheight,

                width = \textwidth,

                xlabel = Channel,

                ylabel = Counts,

                ymin = 0,

                xmin = -5,

                xmax = 132,

                axis on top,

            ]

            % Vertical division of each Board [0-7]

            \foreach \i in {0,...,8} {

                \node (a) at (current axis.above origin) {};

                \node (b) at ($ (a) + (0-0.5,0) $) {};
            \addplot[draw=gray] (16*\i-0.5,0) -- (b);                        
        }                                                                    
        \addplot[                                                            
            ybar interval,                                                   
            x filter/.code=\pgfmathparse{#1-0.5},                            
            fill = black!20] table {data.txt};                               
    \end{axis}                                                               
\end{tikzpicture}                                                            
\caption{Plot.}                                                              

\end{figure}

\end{document}