Introduction to Lattice Graphics and Smooth Lines in R

Lattice graphics is a powerful tool for creating high-quality plots in R. It allows users to create complex plots with multiple layers and customization options. In this article, we will explore how to use lattice graphics to create smooth lines through groups of data points and add them to a multipanel scatterplot.

Setting Up the Data

First, let’s set up our dummy dataframe df as described in the original question:

x <- rep(1:10, 4)
a <- as.factor(rep(1:4, 10))
b <- as.factor(sort(rep(1:2, 20)))
y <- rep(NA, 80)
df <- data.frame(x, y, a, b)
df$y[df$b == "1"] <- df$x[df$b == "1"] + df$x[df$b == "1"]^0.5
df$y[df$b == "2"] <- df$x[df$b == "2"] + df$x[df$b == "2"]^2
for(i in 1:80) df$y[i] <- df$y[i] + rnorm(1, 0, 10)

This will create a dataframe df with four groups (a) and 20 data points per group.

Using Lattice Graphics to Create Smooth Lines

The original question asks how to use lattice graphics to create smooth lines through each panel using a grouping factor. We can do this by creating a custom panel function that calculates the smoothed values for each group:

library(lattice)
my.panel.loess <- function(x, y, span = 2/3, degree = 0.5, ...) {
  loess.fit <- loess.smooth(x, y, span = span, degree = degree)
  panel.lines(loess.fit$x, loess.fit$y, ...)
  panel.xyplot(x, y, ...)
}

xyplot(y ~ x|a, data = df, groups = b, type = "b", 
       panel = function(...) 
         panel.superpose(panel.groups = my.panel.loess, ...))

However, this code does not produce the desired output because we are trying to use a custom panel function that takes multiple arguments (x, y, and other parameters). The lattice graphics system expects panel functions to only take one or two arguments.

Solution Using xyplot with Multiple Arguments

One way to achieve the desired result is by using the xyplot function with multiple arguments in the data argument. This allows us to specify separate panels for each group:

library(lattice)
xyplot(y ~ x|a, data = df, groups = b, type = "b", 
        panel = function(x, y, ...) {
          if (is.numeric(x)) {
            panel.smooth(x, y, ...)
          } else {
            panel.xyplot(x, y, ...)
          }
        })

However, this code will not produce the desired output because it does not calculate the smoothed values for each group.

Solution Using xyplot with Custom Panel Function

Another way to achieve the desired result is by using a custom panel function that calculates the smoothed values for each group:

library(lattice)
my.panel.loess <- function(x, y, span = 2/3, degree = 0.5, ...) {
  loess.fit <- loess.smooth(x, y, span = span, degree = degree)
  panel.lines(loess.fit$x, loess.fit$y, ...)
  panel.xyplot(x, y, ...)
}

xyplot(y ~ x|a, data = df, groups = b, type = "b", 
       panel = function(...) 
         panel.superpose(panel.groups = my.panel.loess, ...))

However, this code does not produce the desired output because we are trying to use a custom panel function that takes multiple arguments.

Solution Using latticeExtra Package

One way to achieve the desired result is by using the latticeExtra package which provides additional functionality for lattice graphics. We can use the xyplot function with the groups argument and specify the type of plot as “p” (points) and “smooth” (smooth line):

library(lattice)
library(latticeExtra)

xyplot(y ~ x|a, data = df, groups = b, type = c("p", "smooth"))

This code produces a multipanel scatterplot with smooth lines through each panel using a grouping factor.

Conclusion

In this article, we explored how to use lattice graphics to create smooth lines through groups of data points and add them to a multipanel scatterplot. We discussed the limitations of the original approach and presented alternative solutions using xyplot with multiple arguments and custom panel functions. Finally, we introduced the latticeExtra package which provides additional functionality for lattice graphics and produced the desired output.

Additional Tips and Tricks

Here are some additional tips and tricks for working with lattice graphics:

• Use the span argument to specify the smoothing parameters (e.g., loess.smooth(x, y, span = 0.7)).
• Use the degree argument to specify the degree of the polynomial used in the smooth line calculation (e.g., loess.smooth(x, y, degree = 2)).
• Use the panel.lines() function to add a line to each panel.
• Use the panel.xyplot() function to plot individual points on each panel.

By following these tips and tricks, you can create high-quality lattice graphics that are informative and easy to understand.

Last modified on 2025-02-28