documentclass[border=3mm]{standalone}
usepackage{tikz}
begin{document}
begin{tikzpicture}
draw [-latex] (0,-0.05) -- (0,6);
draw [-latex] (-0.5,0) -- (6,0);
foreach x in {0,1,2}
  node [below] at (x,0) {x};

node [left] at (0,1) {$psi(a)$};
node [left] at (0,3) {$psi(b)$};

draw (0,3) -- (1,1) -- (2,0);
draw [dashed] (0,1) -| (1,0);

draw (2,0) -- (2,4);
draw [dashed] (2,4) -- (2,5.7);

begin{scope}
clip (2,0) -- (1,1) -- (0,3) -- (0,4) -- (2,4) -- (2,0);
foreach x in {-4,-3.5,...,1.5}
  draw (x,0) -- ++(45:6);
end{scope}
begin{scope}
clip (0,4) rectangle (2,5.7);
foreach x in {-1.5,-1,...,1.5}
  draw [dashed] (x,4) -- ++(45:4);
end{scope}

draw (1.5,2.7) -- ++(2,1) node[right,font=Large] {$Pi$};
end{tikzpicture}
end{document}