'fill' doesn't work properly with 'allow_interruptions'

[gazpachoking]

Using a DateTimeLine graph with the allow_interruptions option doesn't work properly when using a series with fill. When an interruption happens, it completes the shape back to the start of the line rather than down to the x axis.

When an interruption happens, it should draw the line down to the axis the same way it does at the start and end of the graph.