Question

I want to draw ellipses, hyperbolas in R.
How can I do it using ggplot2?
Let me give an example

x^2 +3xy+ 4x+ 2y+ 2y^2 = 0 

for

x between -2 and 2
y between -2 and 2

Answer 1

Construct a data field:

d <- transform(expand.grid(x=seq(-2,2,length=51),
            y=seq(-2,2,length=51)),z=x^2+3*x*y+4*x+2*y+2*y^2)

Make a basic geom_contour plot with each level coloured differently:

g1 <- qplot(x,y,z=z,data=d,colour=factor(..level..),geom="contour")

Now get rid of the lines for all but the “0” level. This depends on stat_contour picking zero as one of its contour levels, which might be fragile under some circumstances … (It’s not obvious to me that stat_contour() allows control of what levels are chosen, equivalent to the levels argument to contour() or contourLines() in base R — if it did, this process would be a little bit easier. One might submit a wishlist item to https://github.com/hadley/ggplot2/issues , if one cared enough about this …)

g1 + scale_colour_discrete(breaks="0",limits=c("0","0"),na.value=NA,
                          guide="none")