在 pgfplot 中的图形上方添加第二个刻度

Question

是否可以在横轴上为同一个图形添加第二个刻度？我想将日期与自参考日期以来的天数结合起来。插图应该可以清楚地说明我的意思：

我目前拥有的代码：

\documentclass{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
\usepgfplotslibrary{dateplot}
\pgfplotsset{compat=1.9}
\usepackage{csvsimple}
\usepackage{filecontents}

\begin{filecontents*}{data.csv}
date,days,value
2015-01-01, 0, 3.2
2015-01-06, 5, 6.5
2015-01-15, 14, 6.8
2015-02-01, 31, 2.2
\end{filecontents*}

\begin{document}
\begin{tikzpicture}
\centering
\begin{axis}[
    xmin=2015-01-01,
    xmax=2015-02-01,
    date coordinates in=x,
    xticklabel={\day-\month-\year},
    x tick label style={rotate=45,anchor=north east},
    grid=both,
    ymin=-1,ymax=10,
    xlabel={Date ($day-month-year$)},
    ylabel={Value},
]
\addplot table [x=date, y=value, col sep=comma] {data.csv};
\end{axis}
\end{tikzpicture}
\end{document}

Answer 1

通过添加带有不可见线的第二个 y 轴找到了答案；但也许有更优雅的方法来做到这一点：

\documentclass{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
\usepgfplotslibrary{dateplot}
\pgfplotsset{compat=1.9}
\usepackage{csvsimple}
\usepackage{filecontents}

\begin{filecontents*}{data.csv}
date,days,value
2015-01-01, 0, 3.2
2015-01-06, 5, 6.5
2015-01-15, 14, 6.8
2015-02-01, 31, 2.2
\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},
    grid=both,
    ymin=-1,ymax=10,
    enlarge x limits=false,
    xlabel={Date ($day-month-year$)},
    ylabel={Value},
]
\addplot table [x=date, y=value, col sep=comma] {data.csv};
\end{axis}
\begin{axis}[
    hide y axis,
    axis x line*=top,
    enlarge x limits=false,
    xlabel={Days since 01-01-2015},
]
\addplot [opacity=0] table [x=days, y=value, col sep=comma] {data.csv};
\end{axis}
\end{tikzpicture}
\end{document}