draw curve on tikz

descretep.tex

\documentclass[dvipdfmx, margin=5pt]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
 \def \gamma{\x, \x * exp(-\x)}
 \fill [fill=gray!30] (0.5,0) -- plot[domain=0:0.5] (\gamma) -- (0.5, 0) -- cycle;
 \fill [fill=gray!30] (2,0) -- plot[domain=2:2.5] (\gamma) -- (2.5, 0) -- cycle;
 \fill [fill=gray!30] (4,0) -- plot[domain=4:5] (\gamma) -- (5, 0) -- cycle;
 \draw[color=black, domain=0:5,  samples=100] plot (\gamma) node[right] {};
 \draw[->, very thin] (0,0) -- (5.1,0) node[right] {};
  \draw (0.25, 0) node[below] {$h_1$};
    \draw (1.25, 0) node[below] {$\cdots$};
 \draw (2.25, 0) node[below] {$h_j$};
     \draw (3.25, 0) node[below] {$\cdots$};
  \draw (4.5, 0) node[below] {$h_m$};
\end{tikzpicture}
\end{document}