This fits to the data, a logarithmic function of the form:

y = q*[logn(x)]+b

where the logarithm base (n) is selected by the user (by default, the natural logarithm log (base e) is selected, where e = 2.718...). The base of the log function can be changed globally on the Documents/Graphs: Settings tab of the Options dialog box or locally on the Options 2 tab of the various graph specification dialog boxes.