How to keep xtick label in pgfplots for those coordinates wihout y value

Question

I construct plots by coordinates from table. For some coordinates, the y value is not given which causes the related xtick label disappears. But I want these xtick labels for such coordinates being shown on my plots. How to do it? Here is my MWE:

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.6}
\usepackage{filecontents} 

\begin{document}
\begin{filecontents}{test.dat} 
time    id  a
59:58   1   1
59:59   2   4
00:00   3   nan
00:01   4   16
00:02   5   25
\end{filecontents}

\begin{tikzpicture}
\begin{axis}[
title={example},
xlabel=time,
ylabel={$data$},
xticklabel style={rotate=90},
xtick=data,
xticklabels from table={test.dat}{time} 
]
\addplot [blue,mark=*] table[x=id,y=a] {test.dat};
\end{axis}
\end{tikzpicture}

\end{document}

Answer 1

For this case you could use xtick={1,...,5} instead of xtick=data.

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.6}
\usepackage{filecontents} 

\begin{document}
\begin{filecontents}{test.dat} 
time    id  a
59:58   1   1
59:59   2   4
00:00   3   nan
00:01   4   16
00:02   5   25
\end{filecontents}

\begin{tikzpicture}
\begin{axis}[
title={example},
xlabel=time,
ylabel={$data$},
xticklabel style={rotate=90},
xtick={1,...,5},
xticklabels from table={test.dat}{time} 
]
\addplot [blue,mark=*] table[x=id,y=a] {test.dat};
\end{axis}
\end{tikzpicture}

\end{document}