Something like this for the graph? http://jsfiddle.net/6sdH6/3/

Highcharts documentation is incomplete. In order to override the tooltip configurations for a given series you need to define the tooltip object inside it.

I also added a different axis for the hours, but this results in a Highcharts bug where a time series point does not show a tooltip when there is a series defined after that one with a higher y value.