I want to add two annotations to the ggplot graph.
When the graph doesn't contain a `facet_grid`, such as `p1`, adding such a `annotate` layer works fine, i.e., `q1`. However, when I add a `facet_grid` layer, to the original graph, i.e., `p2`, then adding the same 'annotate' layer, i.e., `q2` results in an error reporting: 

> 
> Error: Aesthetics must be either length 1 or the same as the data (4): label
> 
> 
> 

Any suggestion? Thanks.

PS, the version of the package ggplot2 I used is 2.2.1.

`p1 <- ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point() 
p2 <- p1 + facet_grid(vs~.)
q1 <- p1 + annotate("text", x = 2:3, y = 20:21, label = c("my label", "label 2"))
q2 <- p2 + annotate("text", x = 2:3, y = 20:21, label = c("my label", "label 2"))`