library(ggplot2)

#example stored in baseplot
ID <- 1:400
values1 <- rnorm(200, mean=50, sd=10) 
values2 <- c(rnorm(100, mean=20, sd=10), rnorm(100, mean=40, sd=10))
df <- data.frame(ID, values1, values2)
baseplot <- ggplot(df, aes(values1, values2)) +
  geom_point(size=2) 
baseplot

#adding the plot title with labs(title="")
baseplot +
  labs(title="values2 vs. values1")

#adding the plot title, a subtitle and a tag
baseplot +
  labs(title="values2 vs. values1", subtitle="presented in the form of a scatterplot", tag="Fig.1A")


#changing the plot title with theme(plot.title = element_text())
baseplot +
  labs(title="values2 vs. values1") +
  theme(plot.title = element_text(family = "serif", 
                                  face = "bold", 
                                  color = "blue",
                                  size = 18,
                                  hjust = .5,
                                  vjust = 2))