Getting used to the grammar of ggplot2 takes some time, but so far it's not been disappointing. Wanting to split a scatterplot by segment, I used facet_grid. That by default shows a label on each subplot, using the values in the variable by which the plot is faceted.

As that often isn't very descriptive in itself, there needs to be a way to re-label these subplots. That way is as_labeller, as shown in the example code below.

Example: 

ggplot(outputs, aes(x = date_var,y = value_var), alpha = 0.8) + 
  geom_point(aes(y = value_var, colour = colour_var)) + 
  geom_smooth() + 
  theme(legend.position = "none",
        axis.text.y = element_blank(),
        axis.text.x = element_blank()) +
  scale_x_date(date_breaks = '1 week') + 
  labs(y = "Value", x = "Date",
  title = "Example") +
  scale_colour_manual("Legend", values = named_coloring_vector)) +
  scale_fill_manual(“", values = c("grey12”)) + facet_grid(. ~ Segment,
  labeller = as_labeller(c("yes" = "Segment A", "no" = "Segment B")))

Output:

Example plot with 2 facets labelled Segment B and Segment A.

Figure 1: Example plot with 2 facets labelled Segment B and Segment A.