Multi line title in ggplot 2 with multiple italicized words

Using a combination of atop, paste, italic and scriptstyle:

ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point() +
  labs(title = ~ atop(paste('First line of title with ',italic("Species")),
                      paste(scriptstyle(italic("Species")),
                            scriptstyle(" secondline words "),
                            scriptstyle(italic("anotherSpecies")),
                            scriptstyle(" the end"))))

gives you the desired result:

Using scriptstyle is not a necessity, but imho it is nicer to have your subtitle in a smaller font than the main title.

See also ?plotmath for other usefull customizations.

We could also call cowplot::draw_label() two times (inspired from this discussion). However, we need to tweak a bit the position and make enough space for the custom title. I gave more explanations about this approach and also using ggplot2::annotation_custom() in ggplot2 two-line label with expression.

library(ggplot2)
library(cowplot)
#> 
#> Attaching package: 'cowplot'
#> The following object is masked from 'package:ggplot2':
#> 
#>     ggsave

# The two lines we wish on the plot. The ~~ creates extra space between the
# expression's components, might be needed here.
line_1 <- expression("First Line of Title with" ~~ italic("Species"))
line_2 <- expression(italic("Species") ~~ "second line words" ~~ italic("anotherSpecies") ~~ "the end")

# Make enough space for the custom two lines title
p <- ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point() + 
  labs(title = "") + # empty title
  # Force a wider top margin to make enough space
  theme(plot.title = element_text(size = 10, # also adjust text size if needed
                                  margin = margin(t = 10, r = 0, b = 0, l = 0,
                                                  unit = "mm")))

# Call cowplot::draw_label two times to plot the two lines of text
ggdraw(p) +
  draw_label(line_1, x = 0.55, y = 0.97) +
  draw_label(line_2, x = 0.55, y = 0.93)

^{Created on 2019-01-14 by the reprex package (v0.2.1)}

As an alternative to inserting line breaks in title, you may use title together with subtitle (available from ggplot 2.2.0). Possibly this makes the plothmathing slightly more straightforward.

p <- ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point() +
  labs(title = expression("First line: "*italic("Honorificabilitudinitatibus")),
       subtitle = expression("Second line: "*italic("Honorificabilitudinitatibus praelongus")*" and more"))
p

---

If you wish the font size to be the same on both lines, set the desired size in theme.

p + theme(plot.title = element_text(size = 12),
          plot.subtitle = element_text(size = 12))

Note that both title and subtitle are left-aligned by default in ggplot2 2.2.0. The text can be centered by adding hjust = 0.5 to element_text above.