Error in pgfplot when big offset is subtracted

Question

I have some data with a huge offset that I want to plot. Im using PGF plots, where it's possible to subtract the offset from a whole row in my data.

This works when the offset isn't too big, and/or the delta between ticks isn't to small. otherwise the plot will be all twisted.

See folowing MWE:

\documentclass[border=4mm]{standalone}
\usepackage{pgfplots,filecontents}

\begin{filecontents*}{data.dat}
X   Y
149038832.9 19
149038835.1 18
149038837.3 17
149038839.5 16
149038841.7 15
149038843.9 14
149038846.1 13
149038848.3 12
149038850.5 11
149038852.7 10
149038854.9 9
149038857.1 8
149038859.3 7
149038861.5 6
149038863.7 5
149038865.9 4
149038868.1 3
149038870.3 2
149038872.5 1
149038874.7 0
\end{filecontents*}

\begin{document}
\begin{tikzpicture}
    \begin{axis}[]
        \addplot table[x expr=(\thisrowno{0}-149038800),y=Y] {data.dat};
    \end{axis}
\end{tikzpicture}
\end{document}

This example was supposed to give a plot with a straight line, but the result are as follows:

EDIT:

Post-processing the data isn't a possibility, because I'm not allowed to make changes to the datafiles.

However, arcording to this(p.5), it's possible to change the mathematical engine from usepackage pgfmath(default) to fp, which should extend the fixed-point arithmetic. I just don't know how to do this.

Answer 1

It is possible to use another math engine in the following manner

\documentclass[border=4mm]{standalone}
\usepackage{pgfplots,filecontents}
\usepackage{xintexpr}

\begin{filecontents*}{data.dat}
X   Y
149038832.9 19
149038835.1 18
149038837.3 17
149038839.5 16
149038841.7 15
149038843.9 14
149038846.1 13
149038848.3 12
149038850.5 11
149038852.7 10
149038854.9 9
149038857.1 8
149038859.3 7
149038861.5 6
149038863.7 5
149038865.9 4
149038868.1 3
149038870.3 2
149038872.5 1
149038874.7 0
\end{filecontents*}

\begin{document}
\begin{tikzpicture}
    \begin{axis}[]
        \addplot table[x expr=({\xinttheiexpr[1]\thisrowno{0}-149038800\relax}),y=Y] {data.dat};
    \end{axis}
\end{tikzpicture}
\end{document}

I explain the syntax:

1. \xinttheiexpr[d] ... \relax does exact computations with scientific numbers or fractions (no limit on number of digits, but concretely works best with up to a few hundred digits per number) then truncates rounds to d digits after decimal mark. So far, apart from square root (by default, with 16 digits of precision; but sqrt(x,P) will have P digits of precision) no non-algebraic functions implemented though. It is also possible to use only macros from package xintfrac, but the expression syntax is easier to use.

2. I needed braces to trick the `TikZ parsing it seems.

Output:

Answer 2

To work around the limited precision and the restriction that you can't preprocess the data file, you could truncate the number at the string level before plotting it. You can do that using by using \pgfplotstablemodifyeachcolumnelement to loop over the rows, and then extracting the rightmost characters using \StrRight{\cell}{6}[\result] (from the xstring package):

\documentclass[border=4mm]{standalone}
\usepackage{pgfplots,filecontents, xstring, pgfplotstable}

\begin{filecontents*}{data.dat}
X   Y
149038832.9 19
149038835.1 18
149038837.3 17
149038839.5 16
149038841.7 15
149038843.9 14
149038846.1 13
149038848.3 12
149038850.5 11
149038852.7 10
149038854.9 9
149038857.1 8
149038859.3 7
149038861.5 6
149038863.7 5
149038865.9 4
149038868.1 3
149038870.3 2
149038872.5 1
149038874.7 0
\end{filecontents*}

\pgfplotstableread{data.dat}\datatable
    \pgfplotstablemodifyeachcolumnelement{X}\of\datatable\as\cell{%
    \StrRight{\cell}{6}[\result]%
    \edef\cell{\result}%
}

\begin{document}
\begin{tikzpicture}
    \begin{axis}[]
        \addplot table[x expr=(\thisrowno{0}-8800),y=Y] {\datatable};
    \end{axis}
\end{tikzpicture}
\end{document}