wip: getting measurements for selected properties/sensors.
Make series group by type.
Remove sensors that belong to deselected property.
Plot sensors of the same unit only.
Store 'checked' status in app model.
Just the programmes tab now.

This is the least number of changes to get it down to just the
programmes tab.
Finally fixed that broken font.

Serif fonts are fine, but not for headlines on screens. Helvetica will
do for now.