If you've not already found it, The Visual Display of Quantitative Information by Edward Tufte (Graphics Press, 1991) talks a lot about the representation of gr

If you've not already found it, The Visual Display of Quantitative Information by Edward Tufte (Graphics Press, 1991) talks a lot about the representation of graphical train schedules by the slope of a line between stations; the intersection of two lines on the graph shows the time and place where two trains meet going in opposite directions. This was in the nineteenth century.