Hi,
From my understanding you are trying to find the Steady state error(SSE) for the step response. Steady state error is the absolute difference between the actual value and the desired value at the steady state i.e assuming the end of the plot in this case. SSE is meaningless for unstable closed loop systems.
Desired value appears to be 0.5 from the plot.
To get the SSE
[y,t] = step(T);
sse = abs(0.5 - y(end));
