Several data repositories on the web were helpful, either because of the data sets available there or merely as a source of inspiration. Among the most helpful were

Wherever specific data sets have been used, detailed attribution is given in the text.

Two resources I relied on more than on any other: the excellent collection of the University ...

Get Gnuplot in Action: Understanding Data with Graphs now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.