Another option,
xmin <- -5
xmax <- 10
n <- 1e3
xs<-seq(xmin,xmax,length=n)
ys<-seq(xmin,xmax,length=n)
f = function(x, y) (x^2+0.7*y^2-1)^3 - x^2*y^3
zs <- outer(xs,ys,FUN=f)
h <- contourLines(xs,ys,zs,levels=0)
library(txtplot)
with(h[[1]], txtplot(x, y))
+---+-******----+----******-+---+
1.5 + ***** ********** ***** +
1 +** * +
0.5 +** * +
| *** *** |
0 + **** **** +
-0.5 + ***** ***** +
-1 + *********** +
+---+-----+-----*-----+-----+---+
-1 -0.5 0 0.5 1
