Let's say I have system A, B, C, and D. Each system contains 10,000 numbers generated by Poisson distribution. The difference is the mean is different for different systems. I calculated the std dev for each distribution corresponding to each system. I plot the standard deviation with respect to mean numbers. (I observe std deviation increases with mean). I want to fit the plot with some line, which gives me the general estimate of std dev with respect to the mean number. I need help in finding the fitting line!