R Changing Order of Facets

As you only have two factor levels you can use forcats::fct_rev() within your ggplot code (no need to load forcats if you have tidyverse already loaded).

library(forcats)
# replace last line of code with
facet_grid( ~ fct_rev(group), scale = "free")

dfx$group <- factor(dfx$group, levels = c("SLG","BA"))