It's not obvious at the moment but you can check historic values through Universe.
Once logged into
Universe navigate to the 'Health' page.
Create a filter for the vehicle you're looking for using the search option:
Note: This can be done without a filter, but it simplifies and quickens the search process.
Clicking the date (directly after 'Diagnostics') brings up the calendar, selecting a date will give you that day's diagnostic state up-time (percentage of the day the health state was reporting active).
For further information, you can click on the group for a popup with a break down of all states in that group:
You can search back as far as Universe has been updated with that vehicle's installation.