I'm having a problem with the following code
\documentclass{standalone}
\usepackage{pgfplots}
\usepgfplotslibrary{fillbetween}
\usetikzlibrary{patterns}
\begin{document}
\begin{tikzpicture}
\begin{axis}[thick, smooth, no markers,
xmin = -1.1, xmax = 1.1,
ymin = -1.1, ymax = 1.1,
axis lines = center,
axis line style={->},
xlabel = $\rho_{S}$,
ylabel = $\rho_{\tau}$,
xlabel style={at=(current axis.right of origin), anchor=west},
ylabel style={at=(current axis.above origin), anchor=south}
]
\addplot[name path = OBEN, samples = 1000, black, domain = -1:1]
{min(1, 0.5+1.5*x)};
\addplot[name path = UNTEN, samples = 1200, black, domain= -1:1]
{max(-1, -0.5+1.5*x)};
\addplot[pattern = north west lines] fill between[of=OBEN and UNTEN];
\end{axis}
\end{tikzpicture}
\end{document}
The axes aren't drawn thick as indicated by the option thick. I get:

instead of something like

Excuse the different zooms levels but it should be clear that I'm not getting thickaxes.