r - ggplot displaying expression in x axis -

below, have r code plots grouped bar plot.

group_name = c('a_1x', 'a_1x', 'a_2x', 'a_2x', 'a_3x', 'a_3x', 'a_4x', 'a_4x')  mydata2 <- data.frame(mygroup = group_name,                        mysubgroup = factor(c("yes", "no"),                                            levels = c("yes", "no")),                        value = c(60,40,90,10,55,45,88,12))   ggplot(mydata2, aes(mygroup, value, fill = mysubgroup)) +    geom_bar(position = "dodge", width = 0.5, stat = "identity")+    coord_flip()  

currently, plot looks below. however, want show expressions in x axis shown in below picture.

i have tried this:

group_name = c(expression(a[1*x]),expression(a[1*x]),                expression(a[2*x]),expression(a[2*x]),                expression(a[3*x]),expression(a[3*x]),                expression(a[4*x]),expression(a[4*x])) 

but gives following error:

error in as.data.frame.default(x[[i]], optional = true) :    cannot coerce class ""expression"" data.frame 

how fix it?

here working example - changed group_name 4 elements instead of 8 , manually added them ggplot expression. issue expression type can't column name data.frame. escapes issue.

library(ggplot2) group_name = c('a_1x', 'a_1x', 'a_2x', 'a_2x', 'a_3x', 'a_3x', 'a_4x', 'a_4x') mydata2 <- data.frame(mygroup = group_name,                        mysubgroup = factor(c("yes", "no"),                                            levels = c("yes", "no")),                        value = c(60,40,90,10,55,45,88,12))  group_name = c(expression(a[1*x]),                expression(a[2*x]),                expression(a[3*x]),                expression(a[4*x]))  ggplot(mydata2, aes(mygroup, value, fill = mysubgroup)) +    geom_bar(position = "dodge", width = 0.5, stat = "identity")+    coord_flip() +   scale_x_discrete(labels=group_name)  # adding labels here