The principles of Tufte's that I always come back to are:\

maximize the data/ink ratio - figure out how to show more data with fewer lines, symbols, colors

Clarify by adding data - show the broad trend but allow a viewer to drill down into specific areas of interest

Here is a good example of both points, 2200 data points coherently graphed: http://www.edwardtufte.com/bboard/q-and-a-fetch-msg?msg_id=0...