combining geom_point and geom_line in one plot

Question

I have two data frames similar to these:

date <- c("2014-07-06", "2014-07-06","2014-07-06","2014-07-07", "2014-07-07","2014-07-07","2014-07-08","2014-07-08","2014-07-08")
TIME <- c("01:01:01", "10:02:02", "18:03:03","01:01:01", "10:02:02", "18:03:03","01:01:01", "10:02:02", "18:03:03")
depth <- c(12, 23, 4, 15, 22, 34, 22, 12, 5)
temp <- c(14, 10, 16, 13, 10, 9, 10, 14, 16)
depth.temp <- data.frame(date, TIME, depth, temp)
depth.temp$asDate<-as.Date(depth.temp$date)

date <- c("2014-07-06", "2014-07-07","2014-07-08") 
meandepth <- c(13, 16, 9) 
cv <- c(25, 9, 20) 
depth.cv <- data.frame(date, meandepth, cv)
depth.cv$asDate<-as.Date(depth.cv$date)

from the first one I have created following plot:

library(ggplot2)
p1 <- qplot(asDate, depth, data=depth.temp, colour=temp, size = I(5), alpha = I(0.3))+ scale_y_reverse()
p1 + scale_colour_gradientn(colours = rev(rainbow(12)))

And from the second one this plot:

p2 <- ggplot(depth.cv, aes(x=asDate, y=meandepth))+ scale_y_reverse()
p2 + geom_line(aes(size = cv))

I want to merge both graphs into one with the points in the back and the line in the front, any suggestions? Note that the points and the line are NOT derived from the same data but from two different data frames.

Answer 1

You can add data to any geom_ independent of what you use in the main ggplot call. For this, I'd skip any data or aesthetic mapping assignments in the main ggplot call and do them all in each respective geom_:

library(scales)

gg <- ggplot()
gg <- gg + geom_point(data=depth.temp, aes(x=asDate, y=depth, color=temp), size=5, alpha=0.3)
gg <- gg + geom_line(data=depth.cv, aes(x=asDate, y=meandepth, size=cv))
gg <- gg + scale_color_gradientn(colours=rev(rainbow(12)))
gg <- gg + scale_x_date(labels=date_format("%Y-%m-%d"))
gg <- gg + scale_y_reverse()
gg <- gg + labs(x=NULL, y="Depth")
gg <- gg + theme_bw()
gg

Answer 2

In my case, for the line plot to show I had to set aes like:

aes(x=asDate, y=meandepth, group=1)

in the geom_line plot