add fake data points to extend the limits of the stat_density2d

[schloerke]

fixes issue #77

[schloerke]

set.seed(0)
x<-data.frame(
  x1=c(rnorm(100),rnorm(1,1)),
  x2=c(rnorm(100),rnorm(1,10)),
  x3=c(rnorm(100),rnorm(1,10)))

pm <- ggpairs(x,
  upper=list(continuous="density"),
  diag=list(continuous='densityDiag'),
  lower=list(continuous='points'))
pm

[schloerke]

fixed tests