R - Draw cases per 100k population - r

Question

I try to draw line COVID cases for each date. I do not have output, the lecturer gave just questions. I solved the question but my problem is the output. It looks weird. Here is the question:
"For the ten countries with the highest absolute number of total deaths, draw the following line graphs to visualize whether epidemic has started to slow down and how the growth rate of new cases/deaths differs across those countries.
a) Number of new cases at each date (absolute number vs per 100.000 population)"
Here is my codes:
library(utils)
COVID_data <-read.csv("https://opendata.ecdc.europa.eu/covid19/nationalcasedeath_eueea_daily_ei/csv", na.strings = "", fileEncoding = "UTF-8-BOM")
#Finding ten countries where the highest absolute total deaths number is
abs_total_deaths <-COVID_data %>%
group_by(countriesAndTerritories) %>%
summarise(abs_total_deaths = sum(deaths)) %>%
arrange(desc(abs_total_deaths))
abs_ten_total_deaths <- c('Italy','France','Germany','Spain','Poland',
'Romania','Czechia','Hungary','Belgium','Bulgaria')
#Calculate new cases by dividing absolute number to 100.000 population
#Draw line for each country
COVID_data %>%
filter(countriesAndTerritories %in% abs_ten_total_deaths) %>%
filter(cases >0) %>%
mutate(new_cases = cases/100000) %>%
ungroup() %>%
ggplot()+
geom_line(aes(x = dateRep, y = new_cases, color = countriesAndTerritories),size=1)+
labs(x="Date",
y="New Cases",
title="New Cases per 100.000 population") +
facet_wrap(~countriesAndTerritories)+
theme_bw()
I will also add a pic of my output. I think my graph is not correct, because the output looks really weird. I can't understand where I make a mistake. If you help me, I'll be appreciated that.
Here is the output:

Answer 1

Looking at Belgium, I get total deaths = 25051 from your data file, which tallies exactly with the data here.
It's obvious that the highest value (by far) for every country occurs "on" the earliest date for the country in the file. Amongst your top ten (I agree with your selection), this is 01Mar2021 for every country apart from Spain, and 28Feb2021 for Spain.
These two facts lead me to conclude (1) your graphs correctly display the data you have asked them to summarise and that (2) you have a data artefact: the first record for each country contains the cumulative total to date, whereas subsequent dates contain data reported "in the previous 24 hours". I use quotes because different countries have different reporting conventions. For example, in the UK (since August 2020) "COVID-related deaths" are deaths from any cause within 28 days of a positive COVID test. Citation
Therefore, to get meaningful graphs, I think your only option is to discard the cumulative data contained in the first record for each country. Here's how I would do that:
library(utils)
library(tidyverse)
COVID_data <-read.csv("https://opendata.ecdc.europa.eu/covid19/nationalcasedeath_eueea_daily_ei/csv", na.strings = "", fileEncoding = "UTF-8-BOM")
# For better printing
COVID_data <- as_tibble(COVID_data)
# Which countries have the higest absolute death toll?
# [I get the same countries as you do.]
top10 <- COVID_data %>%
group_by(countriesAndTerritories) %>%
summarise(TotalDeaths=sum(deaths)) %>%
slice_max(TotalDeaths, n=10) %>%
distinct(countriesAndTerritories) %>%
pull(countriesAndTerritories)
COVID_data %>%
filter(countriesAndTerritories %in% top10) %>%
mutate(
deathRate=100000 * deaths / popData2020,
caseRate=100000 * cases /popData2020,
Date=lubridate::dmy(dateRep)
) %>%
arrange(countriesAndTerritories, Date) %>%
group_by(countriesAndTerritories) %>%
filter(row_number() > 1) %>%
ggplot() +
geom_line(aes(x=Date, y=deathRate)) +
facet_wrap(~countriesAndTerritories)
The critical part that excludes the first data row for each country is
arrange(countriesAndTerritories, Date) %>%
group_by(countriesAndTerritories) %>%
filter(row_number() > 1) %>%
The call to arrange is necessary because the data are not in date order to begin with.
This gives the following plot
which is much more like what I (and I suspect, you) would expect.
The sawtooth patterns you see are most likely also reporting artefacts: deaths that take place over the weekend (or on public holidays) are not reported until the following Monday (or next working day). This is certainly true in the UK.