First define this in an M-file:
function [F] = myfunc(thet)
% help
F = zeros(size(thet));
idx = -pi/2 <= thet & thet <=pi/2;
F(idx) = 80*thet(idx)/pi^2;
idx = pi/2 <= thet & thet <=3*pi/2;
F(idx) = 80/pi*(1 - thet(idx)/pi);
Now from the command line:
>> t = linspace(-pi/2,3*pi/2,1000);
>> plot(t,myfunc2(t),'.')