Drawing a unique figure including a curve with tikz in a specific way

Question

I am interested in drawing the following figure with tikz

I (think) I know how to do this with Bézier curves using \draw with filling, but I want the top curve to be an exact graph of a function that depends on a parameter, like f(x)=cos(kx) or f(x)=exp(kx), where I can adjust the parameter k to later animate my tikz figure. My problem is that I do not know how to use \draw to connect everything together, along with plotting the top curve using an actual function along with the green filling underneath. I would appreciate all help on this.

Answer 1

You can add the points and the plot in the same \draw command.

Something like this:

\documentclass[tikz,border=2mm]{standalone}

% example of a function with a parameter \k
\tikzset{declare function={f(\x,\k)=0.5*cos(2*\x r)+\k;}}

\begin{document}
\begin{tikzpicture}
\draw (0,3) |- (4,0) -- (4,3);
\draw[fill=green!20] plot[samples=101,domain=0:4] (\x,{f(\x,2)}) -- (4,0) -| cycle;
\end{tikzpicture}

\begin{tikzpicture}
\draw (0,3) |- (4,0) -- (4,3);
\foreach[count=\ii]\i in {green,orange,yellow}
  \draw[fill=\i!20] plot[samples=101,domain=0:4] (\x,{f(\x,2.5-0.5*\ii)}) -- (4,0) -| cycle;
\end{tikzpicture}
\end{document}