Errors relating to divide by zero for plot_pdf for given data sets

[ahalwright]

I have wasted a lot of time because of errors relating to division by zero when applying pdf_plot to the given data sets (words, worm, blackouts). I finally have gotten pdf_plot to work by using the suggestion of Charles Martin, but I should not have go to the issues to resolve a problem with the given standard examples for the package. Thus, I strongly request that the code for the package be updated to fix these problems.