In previous work on morphological learning, such as (Chan, 2008), (Zhao and Marcus, 2011) and (Goldwater et al., 2006(Goldwater et al., , 2011, the straight lines on logistic scales are usually interpreted as following Zipf's law (Zipf, 1949).
lognormal distributions with large variance also yield straight lines on both the log-log rank-frequency plot and the CCDF plot.