This graph depicts a saddle point in three-dimensional space, which is a point in the domain of a function that is a local minimum in one direction and a local maximum in another. The depicted function is quadratic in and , and the graph shows the convex and concave nature of the function along different axes.
\documentclass[svgnames]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\begin{document}
\begin{tikzpicture}
\begin{axis}[samples=30,ticks=none,xlabel={$V$},ylabel={$T$},zlabel={$F(T,V)$}]
\addplot3[surf,color=DarkBlue,opacity=0.5,domain=-2:2,faceted color=black] {x^2-y^2};
\end{axis}
\end{tikzpicture}
\end{document}