In certain cases you only want a layer to include certain values of a factor. The `keep_levels()` function allows you to pass character values to be included in the layer. The others are ignored. **NOTE: Denominator calculation is unaffected by this function, see the examples on how to include this logic in your percentages'**
Examples
library(dplyr)
mtcars <- mtcars %>%
mutate_all(as.character)
t <- tplyr_table(mtcars, gear) %>%
add_layer(
group_count(cyl) %>%
keep_levels("4", "8") %>%
set_denom_where(cyl %in% c("4", "8"))
) %>%
build()