how to generate a graph/diagram like Google Analytics’s Visitor Flow?

I thought this was an interesting question, so I made an example alluvial diagram using d3: http://nickrabinowitz.com/projects/d3/alluvial/alluvial.html

And, because d3 is so good at animation, and I thought it would look cool, I made an animated version as well: http://nickrabinowitz.com/projects/d3/alluvial/alluvial-dynamic.html

It doesn’t cover everything you might want, but hopefully it will provide some basis. The large block of code in the beginning is just making fake data – you can replace this with your real data, or load it using d3.json. The expected format is similar to the DOM node structure d3 expects for network graphs:

{
    // list of time slots t1 through tn
    times: [
        // list of t1 nodes
        [
            {
                nodeName: "Node 1",
                id: 1,
                nodeValue: 24332
            },
            // etc ...
        ],
        // etc ...
    ],
    // list of all links
    links: [
        {
            source: 1, // id of source node
            target: 5, // id of target node
            value: 3243
        },
        // ... etc
    ]
}

I hope that’s helpful – this isn’t a typical SO response, and it would likely require a certain amount of work to customize to your needs, but I thought it might be useful.