We can use count and then use ggplot to plot the frequency. Here, we are plotting it with geom_bar (as it is not clear from the OP's post)

library(dplyr)
library(ggplot2)
df %>% 
   count(square_id, Smart_Nsmart) %>%
   ggplot(., aes(x= square_id, y = n, fill = Smart_Nsmart)) + 
         geom_bar(stat = 'identity')