Вы можете попробовать:
library(dplyr)
df <- data.frame("Month_Yr" = c("2016-01","2016-02","2016-03","2016-05","2016-06","2016-07","2017-01","2017-02","2017-03","2017-05","2017-06","2017-07","2018-01","2018-02","2018-03","2018-05","2018-06","2018-07"), "Revenue" = c(1200,7826,11892,11376,9055,5000))
df$year <- substr(df$Month_Yr,0,4)
df$mo <- substr(df$Month_Yr,6,7)
df <- df %>%
arrange(year,mo) %>%
group_by(year) %>%
mutate(cumsum = cumsum(Revenue))
Обновлен ответ.