gganimate: include additional variable other than states level variable or frame in title expression

One dirty solution would be to paste together the variables and make a new one to use in the transition_states:

df <- mutate(df, title_var = factor(paste(x, y, sep="-"), levels = paste(x, y, sep="-")))
# # A tibble: 6 x 3
# x y     title_var
# <int> <chr> <fct>    
# 1     1 a     1-a      
# 2     2 a     2-a      
# 3     3 b     3-b      
# 4     4 d     4-d      
# 5     5 c     5-c      
# 6     6 a     6-a  

Then we could use gsub() in ordet to strip closest_state from the unwanted part, like this:

gsub(pattern = "\\d+-", replacement = "", "1-a") 
"a"

So:

ggplot(df, aes(x, x)) +
  geom_point() +
  labs(title = '{gsub(pattern = "\\d+-", replacement = "", closest_state)}') +
  transition_states(title_var, transition_length = 0.1, state_length = 0.1)

Another possibility, slightly more compact, from the author of gganimate himself, following the issue I opened:

https://github.com/thomasp85/gganimate/issues/252#issuecomment-450846868

According to Thomas:

There are multiple reasons why random columns from the input data cannot be accessed so it is unlikely to get any better than this...