Here comes an idea (I saw it in Flowingdata.com) to graph many time series together in a well-designed and simple way.
The thing is that when you try to put many graphs together, the Y-axis becomes too short and changes are really hard to appreciate.
These kind of graphs can be converted into colour graphs which are much easier to understand. The Y-axis is fixed to a given level for all graphs. If one of the time series goes above that level, a darker area start at that point from the Y=0. Negative numbers can be drawn as reddish.
It is much clearer when a time serie is up or down by looking at colours.