Introduction to Lattice GraphicsRichard Pugh4th December 2012

Agenda

• Overview of Lattice Functions• Creating basic graphics• Panelled Graphics• Grouped Data• Multiple Variables• Writing Panel Functions• Summary

The Data We Will Use

• Something relevant and sector independent • London Tube Performance Data from the TFL

website• Excess Travel Hours by Line

http://data.london.gov.uk/datastore/package/tube-network-performance-data http://en.wikipedia.org/wiki/London_Underground

Overview of Lattice Graphics

Overview of Lattice Graphics

• One of the graphic systems of R (others include “Traditional” and “GGPlot”)

• An implementation of the S+ “Trellis” Graphics

• Written by Deepayan Sarkar, Fred Hutchinson Cancer Research Center

List of Lattice Graphic Functions

Function Description Graph Typexyplot Scatter plot Bivariate

histogram Univariate histogram Univariatedensityplot Univariate density line plot Univariatebarchart Bar chart Univariatebwplot Box and whisker plot Bivariate

qq Normal QQ plot Univariatedotplot Label dot plot Bivariatecloud 3D scatter plot 3D

wireframe 3D surface plot 3Dsplom Scatter matrix plot Data Frame

parallel Multivariate parallel plot Data Frame

Key Function Arguments

Argument Descriptionx Plot definition, typically as a formula

data The data frame used for the graphicsubset Any subsets to be applied to the datapanel Function used to draw data in each “panel”groups Grouping variable for the plot

Type of graph Formula Y axis X axis Z axisUnivariate ~ Y Y - -Bivariate Y ~ X Y X -

3D Z ~ X*Y Y X ZData Frame ~ Data Data - -

Creating Basic Graphics

Panelled Graphics

Specifying Panels

• We can use the vertical pipe symbol “|” in order to specify “panels” to be plotted

• This allows us to create the graphics “by” one or more variables

Panel Variables

• During the plot creation, lattice builds a plot data frame containing the variables to plot

• By this time, any categorical “by” variables need to be factors

• So, either change them to characters or factors beforehand

Panel Ordering!

• If you have a factor variable, the ordering of the panels is in “graph” and not “table” order (!!)

• We can resolve using as.table = TRUE• Note that the converting of characters to

factors uses alphabetical ordering

Grouped Data

Grouped Data

• We can specify groupings within our data in order to plot these groupings separately

• By default, lattice will vary styling of the groups specified

• The key argument is the “groups” argument

Take care!

Controlling Styles

• Best done via the underlying lattice style templates

• Use of par.settings argument which takes a list of styles

• We usually use a basic template and change elements of it

• The show.settings function lets you see the current (or adapted) styles

Multiple Variables

Multiple Variables

• We can specify one or more X or Y axis variable in our lattice formula

• This can overwrite the “groups” input• The “outer” argument controls how the

multiple X/Y variables should be plotted

Writing Panel Functions

Panel Functions

• For each lattice graph, R performs the following actions:• Partitions the data• Draws the graph “outline” (i.e. the “panels”)• Passes the data for each panel into the “panel”

function

• We can overwrite this panel function and supply our own …

Panel Functions

• The default “panel” function for a lattice function is “panel.NameOfFunction”

• Let’s look at panel.xyplot …

Arguments for styling

Jitter & other less used bits

Let’s write our own panel function …….

Now what?

• Now we can insert other functions into a panel function which will then impact each individual plot panel

• We can call:• Lattice “low level functions” (lpoints, ltext, llines, lpolygon etc)

• Existing “panel.” functions that exist for this purpose

Summary

Summary

• This was just a quick overview of lattice graphics

• I hope it was useful• For more info, look in the Lattice book, find

examples online or contact Mango …