move legend title in ggplot2 - r

Question

I have been trying to shift my legend title across to be centered over the legend contents using the guide function. I've been trying to use the following code:
guides(colour=guide_legend(title.hjust = 20))
I thought of trying to make a reproducable example, but I think the reason it's not working has something to do with the above line not matching the rest of my code specifically. So here is the rest of the code I'm using in my plot:
NH4.cum <- ggplot(data=NH4_by_Date, aes(x=date, y=avg.NH4, group = CO2, colour=CO2)) +
geom_line(aes(linetype=CO2), size=1) + #line options
geom_point(size=3) + #point symbol sizes
#scale_shape_manual(values = c(1, 16)) + #manually choose symbols
theme_bw()+
theme(axis.text.x=element_text(colour="white"), #change x axis labels to white.
axis.title=element_text(size=12),
axis.title.x = element_text(color="white"), #Change x axis label colour to white
panel.border = element_blank(), #remove box boarder
axis.line.x = element_line(color="black", size = 0.5), #add x axis line
axis.line.y = element_line(color="black", size = 0.5), #add y axis line
legend.key = element_blank(), #remove grey box from around legend
legend.position = c(0.9, 0.6))+ #change legend position
geom_vline(xintercept=c(1.4,7.5), linetype="dotted", color="black")+ #put in dotted lines for season boundaries
scale_color_manual(values = c("#FF6600", "green4", "#0099FF"),
name=expression(CO[2]~concentration~(ppm))) + #manually define line colour
scale_linetype_manual(guide="none", values=c("solid", "solid", "solid")) + #manually define line types
scale_shape_manual(values = c(16, 16, 16)) + #manually choose symbols
guides(colour=guide_legend(title.hjust = 20))+
scale_y_continuous(expand = c(0, 0), limits = c(0,2200), breaks=seq(0,2200,200))+ #change x axis to intercept y axis at 0
xlab("Date")+
ylab(expression(Membrane~available~NH[4]^{" +"}~-N~(~mu~g~resin^{-1}~14~day^{-1})))+
theme(panel.grid.major = element_blank(), panel.grid.minor = element_blank())+
geom_errorbar(aes(ymin = avg.NH4 - se.NH4, #set y error bars
ymax = avg.NH4 + se.NH4),
width=0.1)
I have tried doing the following instead with no luck:
guides(fill=guide_legend(title.hjust=20)
I have also adjusted the hjust value from values between -2 to 20 just to see if that made a difference but it didn't.
I'll try to attach a picture of the graph so far so you can see what I'm talking about.
I've looked through all the questions I can on stack overflow and to the best of my knowledge this is not a duplicate as it's specific to a coding error of my own somewhere.
Thank-you in advance!!

Answer 1

The obvious approach e.g.
theme(legend.title = element_text(hjust = .5))
didn't work for me. I wonder if it is related to this open issue in ggplot2. In any case, one manual approach would be to remove the legend title, and position a new one manually:
ggplot(mtcars, aes(x = wt, y = mpg, colour = factor(cyl))) +
geom_point() +
stat_smooth(se = FALSE) +
theme_bw() +
theme(legend.position = c(.85, .6),
legend.title = element_blank(),
legend.background = element_rect(fill = alpha("white", 0)),
panel.grid.major = element_blank(),
panel.grid.minor = element_blank()) +
annotate("text", x = 5, y = 27, size = 3,
label = "CO[2]~concentration~(ppm)", parse = TRUE)
Output: