\documentclass[a4paper,12pt]{report}
\usepackage{amsthm,amssymb,amsmath,mathrsfs}
\usepackage{fontspec}
\usepackage{tikz}

\usepackage[arc,all]{xy}
\usepackage[options]{xy}
\usepackage{xepersian}

\settextfont[Scale=1.1]{XB Zar}
\setdigitfont{PGaramond}
\pagestyle{plain}
\begin{document}

\begin{center}
%\usetikzlibrary{arrows}
\begin{tikzpicture}[
    scale=1,
    axis/.style={very thick, ->, >=stealth'},
    important line/.style={thick},
    dashed line/.style={dashed, thin},
    pile/.style={thick, ->, >=stealth', shorten <=2pt, shorten
    >=2pt},
    every node/.style={color=black}
    ]\label{fig:filtration1}
     \begin{scope}
        \shade[inner color=gray!15,outer color=gray!15]
                     (-.6,-.6) rectangle (3,5.7);
    \end{scope}
    %grid
\draw[color=gray] (-0.9,-0.9) grid (5.9,5.9);
% p-q axis
   \draw[axis] (-1,0)  -- (6,0) node(xline)[right]{$p$};
    \draw[axis] (0,-1) -- (0,6) node(yline)[above] {$q$};
    %line n=p+q
  \draw[color=black,thick] (-0.8,5.8) -- (6,-1) node[below right]{$n=p+q$};
  %red points
      \fill[black] (3,2) circle (3pt);
      \fill[black] (2,3) circle (3pt);
       \fill[black] (1,4) circle (3pt);
         \fill[black] (0,5) circle (3pt);
          %line p
           \draw[color=black,thick] (3,6) -- (3,-1) node[ below]{$p$};
       \end{tikzpicture}
\end{center}}
\end{document}