%% MWE for SX: Defining variables with numbers taken from a file
documentclass{standalone}

usepackage[utf8]{inputenx}
usepackage[T1]{fontenc}
%% Use only sans-serif fonts; change to serif if desired
renewcommand*sfdefault{phv}
renewcommand*{familydefault}{sfdefault}
usepackage{arevmath}

usepackage{tikz}
usepackage{pgfplots}
pgfplotsset{compat=newest}
usepgfplotslibrary{units}
usetikzlibrary{spy, backgrounds}
usepackage{pgfplotstable}
usepackage{xcolor}
usepackage{amsmath}
%% To read variables from file
usepackage{datatool}
usepackage{filecontents}

newlengthfigurewidth
newlengthfigureheight
newlengthmarksize

begin{document}
%% Width and height of the output figure, adapt as necessary
setlength{figurewidth}{13cm}
setlength{figureheight}{8cm}
setlength{marksize}{2.4pt}
setlength{linewidth}{1pt}

%% Define the file in here, as whole files cannot be uploaded to Tex.SE
begin{filecontents}{LsqExponents.csv}
4.012,3.456,2.345
end{filecontents}
%% Now read these three values from the file
DTLloaddb[noheader, keys={b_annu, b_rest, b_stra}]{LsqExponents}{LsqExponents.csv}
%DTLdisplaydb{LsqExponents}

begin{tikzpicture}[font=large]

begin{axis}[%
width=figurewidth,
height=figureheight,
scale only axis,
xmin=0.05,
xmax=0.5,
xtick={0.1,0.2,0.3,0.4,0.5},
xlabel={$text{T G S } gamma_mathrm{g} text{ (-)}$},
%xmajorgrids,
ymin=0,
ymax=0.12,
ylabel={$text{G R P } h_mathrm{g} text{ (-)}$},
yticklabel style={/pgf/number format/fixed,
/pgf/number format/precision=2,
/pgf/number format/fixed zerofill},
%ymajorgrids,
name=plot1,
legend pos=north west,
legend style={anchor=north west,draw=black,fill=white,legend cell align=left, rounded corners=2pt, nodes={inner sep=4pt,text depth=0pt}}
]
%% Fits and functions
addplot [color=cyan, solid, domain=0:0.5] {x^4.567};
addlegendentry{Original TD Fit};

addplot [color=magenta, dashed, domain=0:0.5] {x^3.456};
addlegendentry{TD-like Fit for Rest};

addplot [color=black, dashed, domain=0:0.5] {x^2.345};
addlegendentry{TD-like Fit for Stra};

end{axis}

end{tikzpicture}

end{document}