Identify transfer funtion given step response
Well, complementing on the comment by Chu, I would:


Assume that the system is a second order one.
Suggest that, from the plot, actually the initial slope is zero (it seems to smoothly ramp up in the beginning), so there is a two degree difference between numerator and denominator. That means no zero.
Then use standard overshoot and settling time equations for second order responses to derive the damping ratio and natural frequency.
Last, the steady-state gain is unity.