A streamgraph shows the relationship of multiple data series over a period of time. In this case, the production of batters on a baseball team over the course of a season.
Weighted On Base Average is a statistic that measures the offensive production of a batter. It assigns weights to various offensive contributions (i.e. Walk, Double, Home Run), which is then divided by the number of plate appearances.
Since wOBA is a rate statistic, and rates aren't necessarily indicative of total production, this chart uses wOBA * Plate Appearances to show a batter's total offensive contribution.
Each different colored squiggly line represents a player's contribution over the course of the season. Higher production is shown with a taller segment. The total height of all of the layers represents the offensive production of the entire team.
Hover the mouse over the chart to see the name of the player associated with a layer and the 10 day averaged production value leading up to a particular date. Clicking on a layer will make the other layers fade, which can make it easier to see amongst a group of tightly packed layers.