Scatter plot of the frequency drift rate vs frequency for 1389 simple type III bursts; the data points are color-coded according to the occurrence time of the corresponding event. The solid line indicate the least-squares fitting result df/dt = 0.0672 f^1.23 in this work. The dot-dashed, dotted, and dashed lines stand for the results in the literature obtained by Alvarez & Haddock (1973a), Mann et al. (1999), and Melnik et al. (2011), respectively.