Overlay density plot to each existing facet wrapped density plot in ggplot2?

Question

I have a dataframe with ~37000 rows that contains 'name' in string format and 'UTCDateTime' in posixct format and am using it to produce a facet wrapped density plot of time grouped by the names:

I also have a separate density plot of posixct datetime data from an entirely different dataframe:

I want to overlay this second density plot on each individual facet_wrapped plot in the first density plot. Is there a way to do that? In general, if I have plots of any kind that are facet wrapped and another plot of the same type but different data that I want to overlay on each facet of the facet wrap, how do I do so?

Answer 1

This should in theory be as simple as not having the column that you're facetting by in the second dataframe. Example below:

library(ggplot2)

ggplot(iris, aes(Sepal.Width)) +
  geom_density(aes(fill = Species)) +
  geom_density(data = faithful,
               aes(x = eruptions)) +
  facet_wrap(~ Species)

^{Created on 2020-08-12 by the reprex package (v0.3.0)}

EDIT: To get the densities on the same scale for the two types of data, you can use the computed variables using after_stat()*:

ggplot(iris, aes(Sepal.Width)) +
  geom_density(aes(y = after_stat(scaled),
                   fill = Species)) +
  geom_density(data = faithful,
               aes(x = eruptions,
                   y = after_stat(scaled))) +
  facet_wrap(~ Species)

* Prior to ggplot2 v3.3.0 also stat(variable) or ...variable....