You can write the slide in Markdown and HTML. This is bold (markdown) but this is also bold (html).
You might even include inline CSS.
> ggplot(diamonds, aes(carat, price))+geom_point(color="firebrick")
First plot
> ggplot(diamonds, aes(carat, price))+geom_point(color="cadetblue")
Second plot
> ggplot(diamonds, aes(carat, price, color=clarity))+geom_point()
Now I should go back to one column (fullwidth)
This is a paragraph that also gets revealed incrementally.
# this is an R code chunk that will be revealed incrementally
x<-1:10
y<-rnorm(10)