Graphics Reference
In-Depth Information
level has a numeric value of 1, the second level has a value of 2, and so on. You can specify
the numerical intercept manually, or calculate the numerical value using
which(levels(...))
(
Figure 7-9
)
:
pg
<-
ggplot(PlantGrowth, aes(x
=
group, y
=
weight))
+
geom_point()
pg
+
geom_vline(xintercept
=
2
)
pg
+
geom_vline(xintercept
=
which(levels(PlantGrowth$group)
==
"ctrl"
))