# Fill area under a smooth curve

I'm new to tikz and can't find a way to fill the area under this curve. Could someone help me? Here's what I wrote:

\documentclass[tikz]{standalone}

\begin{document}

\begin{tikzpicture}
\draw[-latex, thick] (-.5,0) -- (4,0) node[below] {$x$};
\draw[-latex, thick] (0,-.5) -- (0,4.5) node[left] {$t$};

\draw [cyan!40] plot [smooth] coordinates {(0,3.95) (0.5,3.90) (1,4) (1.5,3.90) (2,4) (2.5,3.90) (3,3.95)};
\draw [cyan!40, ultra thick](0,4) -- (0,0) -- (3,0) -- (3,4);
\end{tikzpicture}

\end{document}


You can "close" your curve by continuing it through the corners in x-axis, and use "fill" in that path.

\documentclass[tikz]{standalone}

\begin{document}

\begin{tikzpicture}
\draw[-latex, thick] (-.5,0) -- (4,0) node[below] {$x$};
\draw[-latex, thick] (0,-.5) -- (0,4.5) node[left] {$t$};

\draw [cyan!40, fill=cyan!20] plot [smooth]
coordinates {(0,3.95) (0.5,3.90) (1,4) (1.5,3.90) (2,4) (2.5,3.90) (3,3.95)}
-- (3,0) -- (0,0) -- cycle;
\draw [cyan!40, ultra thick](0,4) -- (0,0) -- (3,0) -- (3,4);
\end{tikzpicture}

\end{document}


• Is it possible, not to repeat oneself, and to have the ultra thick border included in the first \draw command by using a local scope : {[ultra thick] <remainder of the path>} ? – marsupilam Jun 24 '17 at 20:22