This document introduces the plot() function (short summary here) and xyplot() (found in package lattice).

plot(infmor ~ urb, data=world)
plot(urb, infmor)

As you can see from these two equivalent examples, the function accepts either a formula or two arguments (plot(x,y)). [In both examples urb is the x (horizontal) variable.]

plot(infmor ~ urb, data=world, 
   xlab="Urbanization",ylab="Infant mortality", 
   main="World socio-economic indicators")

In this example you find function arguments adding titles and legends. Further arguments let you for instance change the markers, and colours.

There are several functions to add elements to a graph that is already displayed in the graphics window: title(main="Example illustration the plot function") adds a main title. Other functions let you change the marker type, add lines and smooth curves, as well as adding more points...

The examples below illustrate the most useful functions for our general goal here. Note that most of these functions require x and y coordinates, specified as on plot().

• lines() links all points by drawing a connection line.
• points() adds more points to an existing graph (the function uses the same arguments (options) as plot() and most useful to add points using different markers
• symbols() adds "symbols" to each point, e.g. squares, circles, but also more complex forms, for instance boxplots; the size of the markers can vary depending upon size information supplied.
• text() adds textual elements, thus the sequence

  plot(urb, infmor) 
  text(urb, infmor, labels=rownames(world))
  

  produces a chart where all points are labelled with their respective country abbreviation.

All these functions add elements to the initial plot that already shows the default marker. To avoid messy scatterplots, it often useful to produce a first plot without any marker at all; plot(x,y,type="no") does exactly that..

All graphical functions share common graphical options; these options can be either included as arguments with graphical functions like plot() or be set using the par() function (graphical parameters). Note that the general options are usually only documented in help(par).

Let us illustrate this with an example: each continent will have a different colour.

 plot(urb,infmor,pch=21,  bg = c("red","green","blue","brown","yellow","black") [continent])

• pch=21 specifies the marker type (many types are available using different numbers, however not all are available in colour (type 21 supports colour).
• bg = c("red","green","blue","brown","yellow","black") [continent] bg= (background colour of the marker) In our example we want a different colour for each continent. (the c() function creates a list by combining individual elements; the first element is used for the first continent, the second for the second and so on. The number of colours must of course match the number of continents.

This arguments can of course also be user with functions like points() but of course the interpretation of an argument may vary depending on the type of plot you are producing. Note that if an argument you specify is not used by a particular function, R may or may not warn you that a particular argument is ignored.

Advice: If you intend to produce frequently scatterplots with coloured continents, it is more convenient to store the colour specifications in a variable::

The lattice library offers many additional options, including the possibility to produce conditional plots [coplots]

• xyplot(urb ~ infmor, data=world) will produce the same chart as plot(urb ~ infmor) (but using a different style)
• xyplot(urb ~ infmor | continent, data=world) produces a plot for each continent.
• xyplot(urb~infmor, groups=continent, auto.key=TRUE) produces a scatterplot, with each continent coloured differently; auto.key adds a legend.