Introduction to R - 4th lesson (graphics)

Nathalie Villa-Vialaneix -
September 14-16th, 2015

Master TIDE, Université Paris 1




  • basic concepts
  • graphics parameters
  • graphics devices

Basic concepts on making a plot

  • When making a plot, the user must first choose to what device the plot is sent (a new window, a png file, a postscript file,…) and set the graphic parameters (with par)
  • A plot is made in several steps:
  1. first, a high level graphic function is called (plot, boxplot, pie…) that initiates the graphic;
  2. then, one or several (or even none) low level graphic functions are called (legend, points, lines…) to add elements to the graphic.

Eventually, the graphic can be exported in a file (png, postscript, pdf, …) after.

Graphics parameters

par(bg="lightyellow", mfcol=c(1,2), pch="+")

plot of chunk parGraphics

Graphics parameters

Important graphics parameters include:

  • pch: the plotting symbol (default is an open circle)
  • lty: the line type (default is a solide line)
  • lwd: the line width, specified as an integer multiple
  • col: the plotting color (default is black)
  • las: the orientation of the axis label on the plot (default is 0, which corresponds to labels parallel to the axes, 1 is for horizontal labels, 2 for labels perpendicular to the axes and 3 to vertical labels)
  • bg: the background color (default: white)

Graphics parameters

Important graphics parameters include:

  • mar: the margin size (default is c(5,4,4,2), bottom, left, top, right)
  • oma: the outer margin size (default is 0)
  • mfrow: number of plots per row, column (plots are filled row-wise)
  • mfcol: number of plots per row, column (plots are filled col-wise)

Graphics device

By default graphics are shown on screen but this can be changed using a specific command like png, svg, pdf… Available graphics devices can be found using ?Device.

png("myFistGraphic.png", width=600,
par(bg="transparent", lty=2, lwd=2, 
plot(airquality$Wind, airquality$Ozone) # close the graphics device

Exporting a graphic

A graphics made on the screen can be exported in a graphic device (but this is not an exact operation)

par(bg="transparent", lty=2, lwd=2,
    mar=rep(2,4), col="darkred")
plot(airquality$Wind, airquality$Ozone)

plot of chunk exportGraphics2

dev.print(png, file="mySecondGraphic.png",
          width=600, height=600)

Generic function plot


  • numeric/non numeric variables
  • one or two variables

plot for one numeric variable

This function displays the value of the variable versus its index, as a scatterplot (default) or a line (type="l"):

plot(airquality$Ozone, type="l")

plot of chunk plot1NumVar

… meaningful only for e.g., time series.

plot for one numeric variable

The standard graphics paramaters can be passed to the function (see also argument type in help):

plot(airquality$Ozone, main="Ozone",
     type="b", pch=19, col="darkred")

plot of chunk plot1NumVar2

plot for one non numeric variable

This function displays the barplot of the distribution of the variable:

plot(factor(airquality$Month), main="Month",
     col="pink", lwd=2, las=3, 

plot of chunk plot1nonNumVar

plot for two numeric variables

This function displays the value of the first variable versus the value of the second variable.

plot(airquality$Wind, airquality$Ozone)

plot of chunk plot2NumVar

plot for two variables: one numeric and one non numeric

This function has different behaviors depending on its syntax:

plot(airquality[,3], factor(airquality[,5]))
plot(factor(airquality[,5]), airquality[,3])

plot of chunk plot2nonNumNumVar

plot for two variables: one numeric and one non numeric