You can try
ggplot(df, aes(x = Sepal.Length, y = Sepal.Width)) +
geom_point() +
ggtitle("setosa") +
theme(plot.title = element_text(hjust = 0.5))
A more "hackish"-one could be this hardcoded approach:
ggplot(df, aes(x = Sepal.Length, y = Sepal.Width)) +
geom_point() +
ggtitle("setosa") +
geom_rect(xmin = 4.225, xmax = 5.875 , ymin=4.5, ymax=4.6, fill ="lightgrey") +
coord_cartesian(clip = 'off', expand = 0.05) +
theme(plot.title = element_text(hjust = 0.5, size = 12),
plot.margin = margin(t = 30, r = 20, b = 20, l = 20, unit = "pt"))
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…