[R] combine symbol beta and text in ggplot2 and adjust size

[Juliet Hannah]

For this example:

library(ggplot2)
p <- ggplot(mtcars, aes(x = wt, y=mpg)) + geom_point()
p <- p + scale_x_continuous(expression(beta))
p

How can I include text with the expression. For example, "level of
\beta". Also, how can I increase
the size of the font for this axis label.

Thanks,

Juliet