2.0 Add average trendline to daily cases/recovered/deaths

[Vanethos]

Description
As a user I would like to see the 5 day average trendline for the daily cases, recovered and deaths

Screenshot

[Vanethos]

As far as I can see there is no way to add a trendline to FL_Chart, the proposed solution is to use a stack with both types of widgets

imaNNeo/fl_chart#499