Dates as xticklabels ; number formated dates provided

Question

I would like to plot temporal data and have dates as tick labels. I have seen chapter 4.20 Symbolic Coordinates and User Transformations from pgfplots manual but it does not correspond to my situation because my dates are already formated as numbers. There is a remark to that effect

Remark: This section applies to users who want to have non-standard input coordinates. If you have normal numbers which don't need to be transformed and you like to have special symbols as tick labels, you should consider using the xticklabels (yticklabels) key described on page 224.

So I have read page 224 and following but I could not figure it out.

I reckon something could be done with the pgfplotstable and pgfcalendar packages but once again I fail to find the relevant information.

I ended up using a manual solution which require adjustment whenever the data change and even needed a scaled ticks=false to prevent some overlapping in some specific cases.

How could I have done this cleanly ?

Here is an exemple derived from my current code :

\documentclass{article}
\usepackage{pgfplots}
\usepackage{pgfplotstable}

\begin{document}

\begin{tikzpicture}
\pgfplotstableread{
7.3124563e+005 4
7.3139367e+005 3
7.3165463e+005 2
7.3233467e+005 1
7.3262672e+005 6
7.3279571e+005 5
7.3320909e+005 4
7.3338963e+005 3
7.3385267e+005 2
7.3465509e+005 6
}\mytable

  \begin{axis}[%
    xtick={730852, 731582, 732313, 733043, 733774, 734504},%ugliness here
    xticklabels={2001, 2003, 2005, 2007, 2009, 2011},%and there
    xlabel={Date},
    ylabel={Data},
    scale only axis,
    width=8cm,
    height=5cm,
    axis on top,
    scaled ticks=false]%and here again!

    \addplot table[x index=0,y index=1]{\mytable};

  \end{axis}
\end{tikzpicture}

\end{document}

The time format I use comes from MATLAB datenum and datestr function which is similar to what is used by the pgfcalendar package except that a shift in range is needed. Namely, one unit is one day, fractional part gives the time. 1-Jan-0000 is coded by 0.0 and 7.3188e+005 correspond to 24-Oct-2003 12:45:07.

Answer 1

I thought it might be a simple thing to modify the coordinate transformation to fit your needs.

It turned out to have the following unexpected side-effects:

1. precision. The dateplot lib of pgfplots uses integer arithmetics to shift to date ZERO. This is lost for your application unless someone implements this "properly".
2. changing the trafo will require any user input (like xmin or xtick) in numeric format unless someone implements a solution which accepts both numeric and textual date formats.
3. matlabs output is NOT the same as pgfcalendar expects. One needs to shift the number range. This needs to be done in integer arithmetics, otherwise the limited precision in TeX will lead to a severe degradation of the precision.

As a consequence, I strongly suggest to reconfigure your application to export properly formatted text dates as third column and configure pgfplots to read it together with its date coordinates=x style.

You can post a feature request to implement support for the three mentioned problems above.

I have a running prototype which ignores (reveals) all these problems. If you still want to see it, you can contact me by mail. I will not post it here because it might produce unexpectedly bad results (or perhaps not so unexpected at all).