Please add derivative example

[omry]

I`m trying to build a CPU usage graph, and I can't figure out how to run derivative on the series. can you please improve the doc to include such an example, its by far the most commonly used function in graphite.

[omry]

I got this far, but for some reason the order of the stack is not what appears in the graph definition. any idea?

:graphs:
  cpu_usage:
    title: CPU Usage
    targets:
      system.cpu.iowait:
        !omap
        - :color: orange
        - :derivative:
        - :key: CPU/IO wait
      system.cpu.system:
        !omap
        - :color: red
        - :derivative:
        - :key: CPU/system
      system.cpu.user:
        !omap
        - :color: green
        - :derivative:
        - :key: CPU/user
      system.cpu.idle:
        !omap
        - :color: black
        - :derivative:
        - :key: CPU/idle
    areaMode: stacked
    hosts: ["*"]