This example demonstrates how to use d3.dispatch to coordinate views with shared state. Here, a bar chart and a pie chart show two views of the same data: population by age group for a given state. (For brevity, a legend is omitted.) This example uses two custom events: a load event when data is available, and a statechange event when the displayed state is changed.
Custom events allow loose coupling of components: views can listen for events and update the DOM accordingly, without needing to explicitly tie each view together. When the drop-down menu changes, a statechange event is triggered which causes any interested listeners to be notified.
Each view uses a unique name, such as "bar" or "pie", so that multiple listeners can be notified for a single event. (D3 requires listeners to be named, rather than anonymous, so that it’s easier to remove or inspect registered listeners.) Thus, the "load.menu", "load.bar" and "load.pie" listene