\documentclass{article}
\usepackage{pgfplots}

\usepackage{xepersian}


\begin{document}

\pgfmathdeclarefunction{poiss}{1}{%
  \pgfmathparse{(#1^x)*exp(-#1)/(x!)}%
}

\begin{tikzpicture}
\begin{axis}[every axis plot post/.append style={
  mark=none,domain=0:100,samples=100,smooth}, 
  axis x line*=bottom, 
  axis y line*=left, 
  enlargelimits=upper]
  \addplot {poiss(5)};
  \addplot {poiss(10)};
  \addplot {poiss(20)};
  \addplot {poiss(50)};
\end{axis}
\end{tikzpicture}

\end{document}