The default value of a is 1. The slider for h is non-linear so that when h is near zero it can be set precisely enough on the slider to get good approximations (six significant digits) to the average rate of change at a. The ability to get an enlarged picture around x = a is especially useful when a is -1 or -2. For in this case the values of e^x are so smallthat one cannot see much from the graph unless it is enlarged.