Fix stacked area (ready)

While updating documentation, I found that selection stack wasn't working for combined data views.

For example stack works for simple dataSource like cpuIdle: [{ 'x': timestamp, 'y': value}..]
But not for combined data such as cpuData: [{time: timestamp, idle: value, stolen: value etc}] because of some assumptions in the stack selection implementation.

This pr introduces a sel.flatten for easily performing flattening operations for d3  methods that expect data in a certain format.

Note: Create patch release after merge.