Graphs frontend: sort channels by timestamp

[rossjrw]

I'm pretty sure I made channels execute in reverse frequency order (58943c9) - therefore the hourly channel should be stacked on top of the others, but it's on the bottom:

This makes the graph inaccurate as it's supposed to show the order of stages that the process goes through.

I don't think I can sort the stacked bars on a per-column basis because they're all different chartjs datasets, but I can sort the datasets so that they're in the right order based on what notifier is supposed to do.