group_columns

library(dplyr)
library(magrittr)

counts <- df %>%
  group_by(source, round) %>%
  summarize(Freq = n())

counts2 <- counts %>%
            group_by(source) %>%
            summarise(Freq = sum(Freq)) %>%
            arrange(desc(Freq)) %>% head(10)

counts <- counts %>% filter( source %in% counts2$source)

#"#ff3333", "#00cc66"
########################
colors <- c('#d9b3ff', '#000000', '#7300e6', '#a64dff', '#8000ff', '#330066',
            '#99ccff', '#0039e6', '#3366ff', '#809fff', '#99b3ff', '#e6ecff')

ggplot(counts, aes(x = candidate, y = Freq, fill = source)) +
  geom_bar(position = position_stack(), stat = "identity", width = .7) +
  labs(title = "Frequency by candidate\n", x = "", y = "Frequency\n") +
  theme_light()+
  theme(text = element_text(size=20)) +
  coord_flip() +
  scale_fill_manual(values=colors)