Pdf of arbitary discharge (x)

Hi all,

Is anybody know how to find the probability distribution function of variable x? X here refers to daily discharge which has been recorded 30 years or more (more than 10000 values, totally). I need to plot pdf of variable x and get the graph like the attached file below. (the small rectangles are results of plotting between pdf(x) and x and the different lines are best fits).

I highly appreciate help and assistance. Thanks in advance.