[CovidGraph] Support multiple counties & average

[Vexed01]

What cog is this feature request for?

CovidGraph

Describe the feature request in as much detail as possible

• [ ] Allow for multiple countries to be selected
• [x] Add a 7-day rolling average to cases & deaths
• [x] Allow for viewing of worldwide data

[Vexed01]

I have decided not to support multiple counties because, while at first glance it makes sense to do, in reality, it is like comparing apples with pears. Different countries have different testing availability and uptake, as well as different populations (though the last point could be solved with per capita data).

I am therefore closing this issue as the other two items have been addressed in recent commits.