Spacing on x-axis using ggplot2 discrete scale - r

Question

I've been working with this code for a while but this is the first time I've only had two categories on my x-axis. For some reason, R will not space out the bins and instead bunches them at the beginning. How do I evenly spread them along the x-axis?
xlabels<-c("A","B")
CTplot <- ggplot(CTsum, aes(x=Treatment, y=FI)) +
geom_col(fill="lightsteelblue") +
scale_x_discrete(labels= xlabels)+xlab("")+ylab("")+
theme(panel.background = element_blank(),axis.line = element_line(colour = "black"),axis.text.x = element_text(size=20,color="black"), axis.text.y = element_text(size=20, color="black"), axis.title.x = element_text(size=25), axis.title.y = element_text(size=25))+
geom_errorbar(aes(ymin = FI - meanse, ymax = FI + meanse, width=0.2))+
annotate("text",x=1,y=0.84, label="A",size=5)+
annotate("text",x=2,y=0.62, label="B",size=5)+
annotate("text",x=3,y=0.29, label="",size=5)+
annotate("text",x=4,y=0.26, label="",size=5)+
annotate("text",x=0.65,y=0.5, label="",size=15)
CTplot

Answer 1

When you plot categorical data on the x axis, you are "really" plotting at integer values x = 1, x = 2, etc, but with the text labels used in place of numbers. This is what allows you to put text annotations at x = 1 and x = 2.
However, the bars are bunched at the left because you have added two empty text annotations over to the right (at position x = 3 and x = 4). The plot has expanded right to accommodate them. Since they are empty anyway, you don't need them. Here is the plot without them:
CTplot <- ggplot(CTsum, aes(Treatment, FI)) +
geom_col(fill = "lightsteelblue") +
geom_errorbar(aes(ymin = FI - meanse, ymax = FI + meanse, width = 0.2)) +
scale_x_discrete(labels = xlabels) +
xlab("") +
ylab("") +
theme(panel.background = element_blank(),
axis.line = element_line(colour = "black"),
axis.text.x = element_text(size=20,color="black"),
axis.text.y = element_text(size=20, color="black"),
axis.title.x = element_text(size=25),
axis.title.y = element_text(size=25))+
annotate("text",x=1,y=0.84,label="A",size = 5) +
annotate("text",x=2,y=0.62,label="B",size = 5)
CTplot
And here it is with an empty annotation at x = 4:
CTplot + annotate("text",x = 4, y = 0.29, label = "", size = 5)
To emphasise the point, let's see an empty annotation at x = 20:
CTplot + annotate("text",x = 20, y = 0.29, label = "", size = 5)
As you can see, the x axis has had to expand to accommodate the invisible text annotation at x = 20.
If you want the bars a bit more spread out, you can do something like:
CTplot <- ggplot(CTsum, aes(Treatment, FI)) +
geom_col(fill = "lightsteelblue", width = 0.6) +
geom_errorbar(aes(ymin = FI - meanse, ymax = FI + meanse, width = 0.2)) +
scale_x_discrete(labels = xlabels, expand = c(0.75, 0)) +
xlab("") +
ylab("") +
theme(panel.background = element_blank(),
axis.line = element_line(colour = "black"),
axis.text.x = element_text(size=20,color="black"),
axis.text.y = element_text(size=20, color="black"),
axis.title.x = element_text(size=25),
axis.title.y = element_text(size=25))+
annotate("text",x=1,y=0.84,label="A",size = 5) +
annotate("text",x=2,y=0.62,label="B",size = 5)
CTplot
Data used (approximated from image in OP)
CTsum <- data.frame(Treatment = c("A", "B"), FI = c(0.71, 0.48), meanse = 0.1)