geom_density_ridges requires the following missing aesthetics: y

I have also noticed , if y is a factor this error will not pop up. try this :)

ggplot(graphing_dataframe, 
        aes(x = id, y=as.factor(rating))) +
  geom_density_ridges()

This question seems to be at the top of the list when googling this error message, so I thought I'd chime in: the other reason you can get this error is if you have your x and y switched:

For instance, with the above data (using id instead), this works:

ggplot(graphing_dataframe, 
        aes(x = rating, y = id)) +
  geom_density_ridges()

While this throws the error:

ggplot(graphing_dataframe, 
        aes(x = id, y=rating)) +
  geom_density_ridges()

Seems as if the error message is a bit misleading, according to the documentation you need to add a group aesthetic for numerical variables:

The grouping aesthetic does not need to be provided if a categorical variable is mapped onto the y axis, but it does need to be provided if the variable is numerical.

so adding group = week to ggplot(aes(…)) ought to do the trick. (Just had the same issue myself.)