No line in geom_line plot (missing values in some categories)

General
2

A quick solution is to remove the rows that have NA values in the ch column.

ggplot(a) + 
  geom_bar(aes(x=score, y=count), stat = "identity") +
  geom_point(aes(x = score, y = ch), color="red") +
  geom_line(aes(x = score, y = ch, group = 1), data = na.omit(a), stat = "identity", color="red")