Forget used ticks

Question

This is a follow-up question of pgfplot dateplot discrete dates on axis, which is answered by Peter Grill. The solution for plotting discrete dates is to use only unique dates.

However, when making two plots with corresponding dates on the axis, the dates are already known and thus not plotted in the second plot (see figure). Is there a way to forget the "Ticks Used" after the first plot? (I tried to undefine the macro \tick with: \let\tick=\undefined, but this doesn't work).

\documentclass{standalone}
\usepackage{pgfplots}
\usepackage{etoolbox}
\usepgfplotslibrary{dateplot}
\pgfplotsset{compat=1.12}
\usepackage{filecontents}

\begin{filecontents*}{data.csv}
    date,value
    2015-01-01, 3.2
    2015-01-02, 6.5
    2015-01-04, 6.8
\end{filecontents*}
\begin{document}
    \begin{tikzpicture}
    \centering
    \begin{axis}[
    date coordinates in=x,
    xticklabel={\day-\month-\year},
    x tick label style={rotate=45,anchor=north east},
    date ZERO=2015-01-01,
    grid=both,
    enlarge x limits=false,
    xlabel={Date (day-month-year)},
    ylabel={Value},
    xticklabel={%
        \ifcsdef{Tick Used \tick}{}{%
            \tick%
            \csxdef{Tick Used \tick}{}%
        }%
    },  
    ]
    \addplot table [x=date, y=value, col sep=comma] {data.csv};
    \end{axis}
    \end{tikzpicture}
    \begin{tikzpicture}
    \centering
    \begin{axis}[
    date coordinates in=x,
    xticklabel={\day-\month-\year},
    x tick label style={rotate=45,anchor=north east},
    date ZERO=2015-01-01,
    grid=both,
    enlarge x limits=false,
    xlabel={Date (day-month-year)},
    ylabel={Value},
    xticklabel={%
        \ifcsdef{Tick Used \tick}{}{%
            \tick%
            \csxdef{Tick Used \tick}{}%
        }%
    },  
    ]
    \addplot table [x=date, y=value, col sep=comma] {data.csv};
    \end{axis}
    \end{tikzpicture}
\end{document}

Answer 1

If you don't want to use unique names, you can adapt the solution from How keep a running list of strings and then process them one at a time and keep a list of the the undef that need to be done. So, instead of the

\csxdef{Tick Used \tick}{}%

you instead use

\MarkTickAsUsed{Tick Used \tick}% 

Adding the following to the preamble will then clear this list at the start of every tikzpicture:

\newcommand\ClearTickList{}
\newcommand\MarkTickAsUsed[1]{%
    \xdef\ClearTickList{\ClearTickList\csundef{#1}\endgraf}% Store tick so that we can clear it later
    \csxdef{Tick Used \tick}{}%
}
\AtBeginEnvironment{tikzpicture}{\ClearTickList}

Code:

\documentclass[border=2pt]{standalone}
\usepackage{pgfplots}
\usepackage{etoolbox}
\usepgfplotslibrary{dateplot}
\pgfplotsset{compat=1.12}
\usepackage{filecontents}

%% https://tex.stackexchange.com/questions/14393/how-keep-a-running-list-of-strings-and-then-process-them-one-at-a-time
\newcommand\ClearTickList{}
\newcommand\MarkTickAsUsed[1]{%
    \xdef\ClearTickList{\ClearTickList\csundef{#1}\endgraf}% Store tick so that we can clear it later
    \csxdef{Tick Used \tick}{}%
}
\AtBeginEnvironment{tikzpicture}{\ClearTickList}



\begin{filecontents*}{data.csv}
    date,value
    2015-01-01, 3.2
    2015-01-02, 6.5
    2015-01-04, 6.8
\end{filecontents*}
\begin{document}
    \begin{tikzpicture}
    \centering
    \begin{axis}[
    date coordinates in=x,
    xticklabel={\day-\month-\year},
    x tick label style={rotate=45,anchor=north east},
    date ZERO=2015-01-01,
    grid=both,
    enlarge x limits=false,
    xlabel={Date (day-month-year)},
    ylabel={Value},
    xticklabel={%
        \ifcsdef{Tick Used \tick}{}{%
            \tick%
            \MarkTickAsUsed{Tick Used \tick}%
        }%
    },  
    ]
    \addplot table [x=date, y=value, col sep=comma] {data.csv};
    \end{axis}
    \end{tikzpicture}
    \begin{tikzpicture}
    \centering
    \begin{axis}[
    date coordinates in=x,
    xticklabel={\day-\month-\year},
    x tick label style={rotate=45,anchor=north east},
    date ZERO=2015-01-01,
    grid=both,
    enlarge x limits=false,
    xlabel={Date (day-month-year)},
    ylabel={Value},
    xticklabel={%
        \ifcsdef{Tick Used \tick}{}{%
            \tick%
            \csxdef{Tick Used \tick}%
        }%
    },  
    ]
    \addplot table [x=date, y=value, col sep=comma] {data.csv};
    \end{axis}
    \end{tikzpicture}
\end{document}