# Lattice Graphs

The lattice package, written by Deepayan Sarkar, attempts to improve on base *R* graphics by providing better defaults and the ability to easily display multivariate relationships. In particular, the package supports the creation of *trellis graphs* - graphs that display a variable or the relationship between variables, conditioned on one or more other variables.

The typical format is

*graph_type*(*formula*, data=)

where *graph_type* is selected from the listed below. *formula* specifies the variable(s) to display and any conditioning variables . For example ** ~x|A** means display numeric variable x for each level of factor A. **y~x | A*B **means display the relationship between numeric variables y and x separately for every combination of factor A and B levels. **~x **means display numeric variable x alone.

graph_type |
description |
formula examples |

barchart | bar chart | x~A or A~x |

bwplot | boxplot | x~A or A~x |

cloud | 3D scatterplot | z~x*y|A |

contourplot | 3D contour plot | z~x*y |

densityplot | kernal density plot | ~x|A*B |

dotplot | dotplot | ~x|A |

histogram | histogram | ~x |

levelplot | 3D level plot | z~y*x |

parallel | parallel coordinates plot | data frame |

splom | scatterplot matrix | data frame |

stripplot | strip plots | A~x or x~A |

xyplot | scatterplot | y~x|A |

wireframe | 3D wireframe graph | z~y*x |

Here are some examples. They use the car data (mileage, weight, number of gears, number of cylinders, etc.) from the **mtcars** data frame.

`# Lattice Examples `

library(lattice)

attach(mtcars)

# create factors with value labels

gear.f<-factor(gear,levels=c(3,4,5),

labels=c("3gears","4gears","5gears"))

cyl.f <-factor(cyl,levels=c(4,6,8),

labels=c("4cyl","6cyl","8cyl"))

# kernel density plot

densityplot(~mpg,

main="Density Plot",

xlab="Miles per Gallon")

# kernel density plots by factor level

densityplot(~mpg|cyl.f,

main="Density Plot by Number of Cylinders",

xlab="Miles per Gallon")

# kernel density plots by factor level (alternate layout)

densityplot(~mpg|cyl.f,

main="Density Plot by Numer of Cylinders",

xlab="Miles per Gallon",

layout=c(1,3))

# boxplots for each combination of two factors

bwplot(cyl.f~mpg|gear.f,

ylab="Cylinders", xlab="Miles per Gallon",

main="Mileage by Cylinders and Gears",

layout=(c(1,3))

# scatterplots for each combination of two factors

xyplot(mpg~wt|cyl.f*gear.f,

main="Scatterplots by Cylinders and Gears",

ylab="Miles per Gallon", xlab="Car Weight")

# 3d scatterplot by factor level

cloud(mpg~wt*qsec|cyl.f,

main="3D Scatterplot by Cylinders")

# dotplot for each combination of two factors

dotplot(cyl.f~mpg|gear.f,

main="Dotplot Plot by Number of Gears and Cylinders",

xlab="Miles Per Gallon")

# scatterplot matrix

splom(mtcars[c(1,3,4,5,6)],

main="MTCARS Data")

click to view

Note, as in graph 1, that you specifying a conditioning variable is optional. The difference between graphs 2 & 3 is the use of the layout option to contol the placement of panels.

## Customizing Lattice Graphs

Unlike base R graphs, lattice graphs are not effected by many of the options set in the **par( )** function. To view the options that can be changed, look at **help(xyplot)**. It is frequently easiest to set these options within the high level plotting functions described above. Additionally, you can write functions that modify the rendering of panels. Here is an example.

`# Customized Lattice Example`

library(lattice)

panel.smoother <- function(x, y) {

panel.xyplot(x, y) # show points

panel.loess(x, y) # show smoothed line

}

attach(mtcars)

hp <- cut(hp,3) # divide horse power into three bands

xyplot(mpg~wt|hp, scales=list(cex=.8, col="red"),

panel=panel.smoother,

xlab="Weight", ylab="Miles per Gallon",

main="MGP vs Weight by Horse Power")

## Going Further

Lattice graphics are a comprehensive graphical system in their own right. Deepanyan Sarkar's book Lattice: Multivariate Data Visualization with R is the definitive reference. Additionally, see the Trellis User's Guide. Dr. Ihaka has created a wonderful set of slides on the subject. An excellent early consideration of trellis graphs can be found in W.S. Cleveland's classic book Visualizing Data.

## To Practice

Try this interactive course on data visualization which covers lattice graphs.