This is cool chart #3.
Charts are frequently used to display volume or size over time. A usual approach is to do this via an area chart in which a data series is displayed on the Y axis and time is displayed on the X axis. Multiple series are stacked and the height of the chart at any point is the total volume at that point. This is sometimes called a river chart.
This is another great interactive graphic from the Flash programmers at the New York Times. It contains a stacked area chart of box office receipts for individual movies. The chart scrolls across the page and covers years 1986 to 2008. Notice that not all streams are labeled. You can click on a particular stream to display the movie mapped to that stream along with an outline of the entire stream.
I like this chart for 2 reasons. First, the Y axis is mirrored (e.g., 0 to n on the top and 0 to n on the bottom) so that streams can be layered on either side of the origin. This technique centers the flow and makes it look more like a river. Second, a spline smoother is used to smooth the lines, visually making it look like a flowing process.