This is the code that I tried but it was just returning the #1 seller for each month, not the best ranking for each publisher for each month.
plt1.dat <- Overall.Sales %>%
group_by(Publisher2, Date) %>%
filter(Rank.in.Units == 1L) %>%
ungroup()
EDIT:
I got it to work!! The 1L was throwing it off! This code works:
plt1.dat <- Overall.Sales %>%
group_by(Publisher2, Date) %>%
filter(Rank.in.Units == min(Rank.in.Units)) %>%
ungroup()