You're right. This doesn't show the artifact I have in the more complex code I'm actually using. It shows another one: the line should be the 7-day average of the columns, but it clearly isn't.
Here's slightly modified code with better labels. I provided two options for defining df, once is just the last of the four regions shown in the other. Note how in the case of just one region, it seems to be correct, that is, the average line is really the average.
covid_reg <- read_csv("https://raw.githubusercontent.com/pcm-dpc/COVID-19/master/dati-regioni/dpc-covid19-ita-regioni.csv")
startDate = "2020-01-01"
df <- filter(covid_reg, denominazione_regione %in% c("Veneto", "Lombardia","Emilia-Romagna","Piemonte") & data >= startDate)
df <- filter(covid_reg, denominazione_regione %in% c("Veneto") & data >= startDate)
ggplot(df, aes(x = data, y = nuovi_positivi)) +
geom_col(width = .75, aes(color = "daily")) +
geom_line(aes(color = "average", y = rollmean(x = nuovi_positivi, k = 7, align = c("right"), fill = NA) ) ) +
facet_wrap (~ denominazione_regione, ncol=4)