Stepwise function in Matlab

Hello , I have a question and would be very grateful if someone can help me ; I have a set of data , as attached , having stepwise trend I want to find the best function to approximate and fit the data .Can you please guide me about the way that I can fit my data by a stepwise function?is it possible to fit the stepwise data by an exponential function like y=a*exp(-b*x) and find the optimum value of a and b for each interval ?Help me in coding that ?

Thanks in advance