How to draw a tangent line on a step response diagram

When I get the step response by sisotool like pic 1m how can I get the tangent line and the value of L, T in pic 2?

[Yubin ZHONG]

Pic 1 lost

Answer 1

[Samatha Aleti]

You can draw a tangent to a step response curve at a particular point as follows:

sys = tf([8 18 32],[1 6 14 24]); % Transfer function  
[y,t] = step(sys); % Step response 
plot(t,y); 
hold on 
slope = diff(y)./diff(t); 
k = 100; % To draw tangent at (100)th point 
yTangent = (t-t(k))*slope(k)+y(k); % Equation of tangent 
plot(t,yTangent); 
scatter(t(k),yTangent(k)); 
hold off