Draw a quadratic spline through points in lattice

Question

Based on this great question: How to draw a smooth curve passing through some points

How would one do this in lattice?

plot(rnorm(120), rnorm(120), col="darkblue", pch=16, xlim=c(-3,3), ylim=c(-4,4))
points(rnorm(120,-1,1), rnorm(120,2,1), col="darkred", pch=16)
points(c(-1,-1.5,-3), c(4,2,0), pch=3, cex=3)
xspline(c(-1,-1.5,-3), c(4,2,0), shape = -1)

Here is similar data, formatted more appropriately for a lattice plot:

dat <- data.frame(x=c(rnorm(120), rnorm(120,-1,1)),
                  y=c(rnorm(120), rnorm(120,2,1)),
                  l=factor(rep(c('B','R'),each=120))
)
spl <- data.frame(x=c(-1,-1.5,-3), 
                  y=c(4,2,0)
)

And here is what the linked question gave, translated to lattice:

xyplot(y ~ x,
       data=dat,
       groups=l,
       col=c("darkblue", "darkred"),
       pch=16,
       panel = function(x, y, ...) {
         panel.xyplot(x=spl$x, y=spl$y, pch=3, cex=3)
         ## panel.spline(x=spl$x, y=spl$y)              ## Gives an error, need at least four 'x' values
         panel.superpose(x, y, ...,
                         panel.groups = function(x, y, ...) {
                           panel.xyplot(x, y, ...)
                         }
         )
       },
       xlim=c(-3,3), ylim=c(-4,4)
)

Answer 1

Here's a line-for line 'translation' of the base graphics solution into lattice. (The directness of the translation is made possible by the + operator supplied by the latticeExtra package. See ?layer for details of its usage.)

The final line invokes grid.xspline(), an exact grid analogue of the base graphic function xspline().

library(lattice)
library(grid)
library(latticeExtra)

xyplot(rnorm(120)~rnorm(120), pch=16, col="darkblue", 
       xlim = c(-3.1, 3.1), ylim = c(-4.1, 4.1)) +
xyplot(rnorm(120,2,1) ~ rnorm(120,-1,1), pch=16, col="darkred") +
xyplot(c(4,2,0) ~ c(-1,-1.5,-3), pch=3, cex=3) +
layer(grid.xspline(c(-1,-1.5,-3), c(4,2,0), shape = -1, default.units="native"))

(One peculiar detail of grid does pop up in the final line above: like several other of its low-level line-drawing functions, grid.xspline() defaults to "npc" units instead of the usually-desired "native" units used as defaults by grid.points() and many other grid.*() functions. Obviously that's easy enough to change --- once you're aware of it!)

Answer 2

This is a little bit tricky but works.

plot(rnorm(120), rnorm(120), col="darkblue", pch=16, xlim=c(-3,3), ylim=c(-4,4))
points(rnorm(120,-1,1), rnorm(120,2,1), col="darkred", pch=16)
points(c(-1,-1.5,-3), c(4,2,0), pch=3, cex=3)

I use xspline without producing the draw

dd <- xspline(c(-1,-1.5,-3), c(4,2,0), shape = -1,draw=FALSE)

Then I use the points produced witn panel.lines

library(lattice)
xyplot(y ~ x,
       data=dat,
       groups=l,
       col=c("darkblue", "darkred"),
       pch=16,
       panel = function(x, y, ...) {
         panel.xyplot(x=spl$x, y=spl$y, pch=3, cex=3)
         panel.lines(dd$x,dd$y)
         panel.superpose(x, y, ...,
                         panel.groups = function(x, y, ...) {
                           panel.xyplot(x, y, ...)
                         }
         )
       },
       xlim=c(-3,3), ylim=c(-4,4)
)