Click/tap once on a country label below the plot to highlight the corresponding curve in black; click a second time to hide; click a third time to restore default colour. I don't know why the graph only zooms once (at most) on Android Chrome (and maybe other browsers?).
Data is sourced from Our World in Data, which currently uses Johns Hopkins University as its source, having (until 30 November 2020) used the European Centre for Disease Prevention and Control.
"New cases" and "New deaths" are last-seven-day averages. Click/tap on "Show top 15 countries" to display the curves for the countries with the highest values for the currently-plotted variable (countries with population less than 1 million are excluded if the current variable is per capita).
updated 2020-03-23 (colours and labels),
updated 2020-03-24 (zoom),
updated 2020-04-10 (moving averages),
updated 2020-04-15 (per capita),
updated 2020-05-13 (7-day rather than 5-day moving averages),
updated 2020-05-31 ("set top 15" option, last-7-day averages),
updated 2020-11-30 (changed data source).