How do I add the gradient color with variables other than x or y?

[Byronxy]

Hi I see example like this

ggplot(lincoln_weather, aes(x = `Mean Temperature [F]`, y = Month, fill = stat(x))) +
  geom_density_ridges_gradient(scale = 3, rel_min_height = 0.01) +
  scale_fill_viridis_c(name = "Temp. [F]", option = "C") +
  labs(title = 'Temperatures in Lincoln NE in 2016')

But I wish to use other variables (continous) to replace the x here, while it does not work

[sohcahtoaster]

I'm having the same issue- let us know if you found an answer!