how i use Low Pass Filter in a rectangle graphic

k=[1 zeros(1,40)];

x=[-200*pi -200*pi 200*pi 200*pi -200*pi ]

y=[0 200*pi 200*pi 0 0]

line(x,y)

axis([-1000 1000 0 5000]);

i have this rectangle and how i use low pass filter in this rectangle ? frequency,bandwidth not important for me ,you can example value these.. Regards..