Add filter by date range

[Roriz]

Feature request

Some countries have a delay in infection, an option to make possible to choose the start date is very important to demonstrate the proportion of numbers.

[fwip]

I would also find this useful.

Further reasoning: It can be sometimes difficult to compare the rate of change in two countries that have different "start times". Some websites attempt to normalize this by using relative start dates for each country/region. For example: https://www.visualcapitalist.com/infection-trajectory-flattening-the-covid19-curve/ - this one represents the X-axis as "days since the country had 100 infections," which allows you to compare the trajectory of each country.

[abshakekumar]

For the issue, i have opened a PR #30 Please see https://github.com/trekhleb/covid-19/pull/30#issuecomment-607492238