dt = 0.01;  % idoleptek
t = 0:dt:10; % idovektor
x = sin(t); % a jel
dx_dt = diff(x)/dt; % a jel derivaltja (elvileg: cos(t))
plot(t, x, t(1:end-1), dx_dt); % abrazolas
legend('x', 'dx/dt'); % jelmagyarazat
xlabel('t'); % tengelyfelirat