Figure 5 of Poisson Distributions

Probability distributions for the difference of independent Poisson-distributed variables. Shown for two values of m1 and m2. Note the skewness and reflection symmetry when m1 and m2 are unequal.

m1 = 1.1, m2 = 1.1

m1 = 1.1, m2 = 4.1

m1 = 4.1, m2 = 1.1

m1 = 4.1, m2 = 4.1