I would like to increase the font size of ggtitle and also the font should be bold. My code is as follows.
ggplot(df, aes(x1, y = value, colour = variable)) + geom_point(size=2) + ggtitle("male vs.female") + theme(axis.text=element_text(size=14), axis.title=element_text(size=14,face="bold")) + theme(legend.text=element_text(size=12)) + labs(x = "x axis", y = "y axis") + ylim(0,100) + xlim(0,100) + scale_colour_manual(values = c("red", "blue"), labels = c("male", "female")) Use theme(), here is an example :
ggplot(cars, aes(x=speed,y=dist)) + ggtitle("cars") + geom_point() + theme(plot.title = element_text(size = 40, face = "bold")) 
Inspired by this answer.
theme that sets a plot.title size, make sure to insert the theme(plot.title = element_text(size = 40, face = "bold")) line after your call to theme_blabla(), otherwise your theme's plot title size will override it.
plot.title not title.You can also use ggtext::element_markdown to change title font size. It provides markdown and HTML rendering for ggplot2. So you can use inline HTML styling to change font size. For example, to make the title 40pt in bold, we can use the following:
ggplot(cars, aes(x=speed, y=dist)) + ggtitle("<b style='font-size:40pt;'>Cars</b>") + geom_point() + theme(plot.title = ggtext::element_markdown()) 
