# [Tex/LaTex] How to get the domain for the line of best fit w/pgfplots to be all real numbers

pgfplotspgfplotstable

Although I already worked around this problem by finding the x-intercept with the linear regression equation I calculated w/gnuplot, I'm still curious to know how pgfplots can do it without me manually computing it.

\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern,tikz,pgfplots,pgfplotstable}

\begin{document}

\begin{center}
\begin{tikzpicture}
\begin{axis}[axis on top=false, axis x line=middle, axis y line=middle,xlabel=$\mathrm{\frac{1}{[S]}}$,ylabel=$\mathrm{\frac{1}{\textit{V}_0}}$,
xmin=-10000,xmax=10000,ymin=-0.01,ymax=0.035]
{
X        Y
10000    0.030
5000     0.02
2000     0.014
1000     0.012
500      0.0110
200      0.0104
100      0.0102
50       0.010
20       0.01
10       0.01
5        0.01
-4999.85 0
-9995.35 -0.01
};
\xdef\slope{\pgfplotstableregressiona}
\xdef\slope{\pgfplotstableregressionb}
\end{axis}
\end{tikzpicture}
\end{center}

\end{document}


The linear regression line is only valid for the domain of the given data, and hence by default it is only drawn from the minimum and maximum x in the given data.

If you want to extrapolate for points outside the domain of the data, you can use \pgfplotstableregressiona and \pgfplotstableregressionb to draw the best fit line and specify the desired domain.

\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern,tikz,pgfplots,pgfplotstable}

\begin{document}
\begin{center}
\begin{tikzpicture}
\begin{axis}[
axis on top=false, axis x line=middle, axis y line=middle,
xlabel=$\mathrm{\frac{1}{[S]}}$,
ylabel=$\mathrm{\frac{1}{\textit{V}_0}}$,
xmin=-10000,xmax=10000,ymin=-0.01,ymax=0.035]
{
X        Y
10000    0.030
5000     0.02
2000     0.014
1000     0.012
500      0.0110
200      0.0104
100      0.0102
50       0.010
20       0.01
10       0.01
5        0.01
%    -4999.85 0     % Assuming that these were manually entered
%    -9995.35 -0.01
};
\xdef\slope{\pgfplotstableregressiona}
\xdef\yintercept{\pgfplotstableregressionb}
\end{axis}
\end{tikzpicture}
\end{center}
\end{document}


I commented out the last two lines that I assume you manually added, and corrected the second \xdef as that is the y-intercept and not the slope.