ribbon

# quick example of creating a plot like that at the link below
# http://www.visualisingdata.com/index.php/2015/01/make-grey-best-friend/

x<-1:10
y<-x^2
z<-x
radius<-x/5
dat<-data.frame(x=x, y=y, z=z, radius=radius)
ggplot(dat, aes(x,y))+ 
  geom_ribbon(aes(ymin=y-radius, ymax=y+radius), alpha=0.5, fill="grey", color="grey", size=1)+
  geom_point(aes(size=z), shape=21, color="grey50", fill="red")