Tikz string diagram illustrating the unit of adjunction

cup-string.txt

\begin{tikzpicture}

\def \xrightmost {2}
\def \xright     {1}
\def \xmid       {0}
\def \xleft      {-\xright}
\def \xleftmost  {-\xrightmost}

\def \ybot       {0}
\def \ymid       {1}
\def \ytop       {2}
\def \ylabel     {\ytop - 0.3}

% functor labels
\node [above] at (\xleft, \ytop)   {$L$};
\node [above] at (\xright, \ytop) {$R$};
% background
\filldraw[fill=orange!30, draw=white] (\xleftmost, \ytop) rectangle (\xrightmost, \ybot);
% cup
\draw [fill=blue!50!green!20] (\xleft, \ytop) to [out=-90, in=180] (\xmid, \ymid) to [out=0, in=-90] (\xright, \ytop);
% natural transformation
\filldraw [black] (\xmid, \ymid) circle (1 pt);
\node [below] at (\xmid, \ymid) {$\eta$};
% category labels
\node           at (\xmid, \ylabel)        {$\mathcal{C}$};
\node [right] at (\xleftmost, \ylabel) {$\mathcal{D}$};

\end{tikzpicture}