Question

I’m trying to plot some data with 2d density contours using ggplot2 in R.

I’m getting one slightly odd result.

First I set up my ggplot object:

p <- ggplot(data, aes(x=Distance,y=Rate, colour = Company))

I then plot this with geom_points and geom_density2d. I want geom_density2d to be weighted based on the organisation’s size (OrgSize variable). However when I add OrgSize as a weighting variable nothing changes in the plot:

This:

p+geom_point()+geom_density2d()

Gives an identical plot to this:

p+geom_point()+geom_density2d(aes(weight = OrgSize))

However, if I do the same with a loess line using geom_smooth, the weighting does make a clear difference.

This:

p+geom_point()+geom_smooth()

Gives a different plot to this:

p+geom_point()+geom_smooth(aes(weight=OrgSize))

I was wondering if I’m using density2d inappropriately, should I instead be using contour and supplying OrgSize as the ‘height’? If so then why does geom_density2d accept a weighting factor?

Code below:

require(ggplot2)

 Company <- c("One","One","One","One","One","Two","Two","Two","Two","Two")
 Store <- c(1,2,3,4,5,6,7,8,9,10)
 Distance <- c(1.5,1.6,1.8,5.8,4.2,4.3,6.5,4.9,7.4,7.2)
 Rate <- c(0.1,0.3,0.2,0.4,0.4,0.5,0.6,0.7,0.8,0.9)
 OrgSize <- c(500,1000,200,300,1500,800,50,1000,75,800)

 data <- data.frame(Company,Store,Distance,Rate,OrgSize)

 p <- ggplot(data, aes(x=Distance,y=Rate))

 # Difference is apparent between these two
 p+geom_point()+geom_smooth()
 p+geom_point()+geom_smooth(aes(weight = OrgSize))

 # Difference is not apparent between these two
 p+geom_point()+geom_density2d()
 p+geom_point()+geom_density2d(aes(weight = OrgSize))

Answer 1

geom_density2d is “accepting” the weight parameter, but then not passing to MASS::kde2d, since that function has no weights. As a consequence, you will need to use a different 2d-density method.

(I realize my answer is not addressing why the help page says that geom_density2d “understands” the weight argument, but when I have tried to calculate weighted 2D-KDEs, I have needed to use other packages besides MASS. Maybe this is a TODO that @hadley put in the help page that then got overlooked?)