R remove two sides of border from strip.background function - r

Question

Dataframe
df <- data.frame(
structure(list(biological_group = structure(1:15, .Label = c("A",
"B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N",
"O"), class = "factor"), norm_expression = c(2L, 3L, 4L, 6L,
1L, 5L, 7L, 8L, 9L, 3L, 2L, 6L, 7L, 8L, 1L), SE = c(0.171499719,
0.089692493, 0.153777208, 0.188012958, 0.153776128, 0.192917199,
0.224766056, 0.231338325, 0.121716906, 0.094763028, 0.09635363,
0.069986333, 0.113681329, 0.094614957, 0.391182473), Group = structure(c(1L,
1L, 3L, 3L, 3L, 3L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("",
"Plant Products", "Sugars"), class = "factor")), class = "data.frame", row.names = c(NA,
-15L), .Names = c("biological_group", "norm_expression", "SE",
"Group")))
Sample code
library(ggplot2)
DFplot <- ggplot(df, aes(biological_group,norm_expression)) +
ylab("Relative Normalized\nExpression (∆∆Cq)") +
geom_bar(fill="black",stat="identity") +
theme(axis.title.x = element_blank())
DFplot2 <- DFplot+geom_errorbar(aes(ymin=norm_expression-SE,ymax=norm_expression+SE),width=0.5) +
geom_boxplot() +
facet_grid(~Group, scales = "free_x", switch = "x", space = "free_x") +
scale_y_continuous(expand = c(0,0), labels = scales::number_format(accuracy = 0.1)) +
theme_classic()
That gives me this graph:
I'd like to remove the vertical lines from the strip text labels (specified by strip.background), like this:
I realize I could just use photoshop or something, but I have several graphs to make so it would be easier if I could just specify it in the code.

Answer 1

This answer was helpful.
In your case, you want to find the strip-b for your bottom strips to substitute.
Edit: Replaced top bar of bottom strip that was removed accidentally.
library(grid)
q <- ggplotGrob(DFplot2)
lg <- linesGrob(x=unit(c(1,0),"npc"), y=unit(c(1,1),"npc"), gp=gpar(col="black", lwd=2))
for (k in grep("strip-b",q$layout$name)) {
q$grobs[[k]]$grobs[[1]]$children[[1]] <- lg
}
grid.draw(q)