[R] scatter plot & equation

[Duncan Murdoch]

On 22/11/2009 11:27 AM, Rofizah Mohammad wrote:
> Hi,
> 
> If I have 2D data set say (x,y) and I can do scatter plot by using plot(x,y)
> command.
> How can I add in this scatter plot the equations curve say
> 
> 2X2 + 3Y2 – 6X – 7Y + 9 = 0.

You could do it using contour(), but you should use an equation that has 
some real solutions.  For example, using a different equation than yours:

x <- rnorm(100, sd=1)
y <- rnorm(100, sd=1)
xgrid <- seq(min(x), max(x), len=100)
ygrid <- seq(min(y), max(y), len=120)
grid <- expand.grid(x=xgrid, y=ygrid)
LHS <- function(x, y) x^2 + y^2 - x - y  - 1
z <- apply(grid, 1, function(x) LHS(x[1], x[2]) )
z <- matrix(z, 100, 120)
plot(x,y)
contour(xgrid, ygrid, z, levels=0, add=TRUE)