How can I get the first abscissa value in decimal form as 0.05 in (0.05,31.9)?
It is set to the scientist notation automatically.
\documentclass[border=2mm]{standalone}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{pgfplots}
\usepackage{siunitx}
\pgfplotsset{compat=1.11}
\begin{document}
\begin{tikzpicture}
\begin{axis}[grid style=dashed,grid=both,ymin=0,ymax=70,xmax=0.35,xmin=0,
minor tick num=1,axis lines = middle,xlabel=$x_1$,ylabel=T\;$(\si{\celsius})$,
x label style={at={(axis description cs:1.1,0.03)},anchor=north},
y label style={at={(axis description cs:0,1.0)},anchor=south},
]
\addplot+[color=red,mark=*,mark options={draw=red,fill=green},smooth]
coordinates {
(0.05,31.9)(0.10,30.1)(0.20,35.8)(0.31,65.4)
};
\end{axis}
\end{tikzpicture}
\end{document}
pgfplots
to recent version (1.16). with it you can write:x label style={at={(1,0)},anchor=west}, y label style={at={(0,1)},anchor=south}