# Combining Plots

R makes it easy to combine multiple plots into one overall graph, using either the
par( )
or layout( ) function.

With the par( ) function, you can include the option mfrow=c(nrows, ncols) to create a matrix of nrows x ncols plots that are filled in by row. mfcol=c(nrows, ncols) fills in the matrix by columns.

```# 4 figures arranged in 2 rows and 2 columns attach(mtcars) par(mfrow=c(2,2)) plot(wt,mpg, main="Scatterplot of wt vs. mpg") plot(wt,disp, main="Scatterplot of wt vs disp") hist(wt, main="Histogram of wt") boxplot(wt, main="Boxplot of wt")```

```# 3 figures arranged in 3 rows and 1 column attach(mtcars) par(mfrow=c(3,1)) hist(wt) hist(mpg) hist(disp)```

The layout( ) function has the form layout(mat) where
mat is a matrix object specifying the location of the N figures to plot.

```# One figure in row 1 and two figures in row 2 attach(mtcars) layout(matrix(c(1,1,2,3), 2, 2, byrow = TRUE)) hist(wt) hist(mpg) hist(disp)```

Optionally, you can include widths= and heights= options in the layout( ) function to control the size of each figure more precisely. These options have the form
widths= a vector of values for the widths of columns
heights= a vector of values for the heights of rows.

Relative widths are specified with numeric values. Absolute widths (in centimetres) are specified with the lcm() function.

```# One figure in row 1 and two figures in row 2 # row 1 is 1/3 the height of row 2 # column 2 is 1/4 the width of the column 1 attach(mtcars) layout(matrix(c(1,1,2,3), 2, 2, byrow = TRUE),    widths=c(3,1), heights=c(1,2)) hist(wt) hist(mpg) hist(disp)```

See help(layout) for more details.

## Creating a figure arrangement with fine control

In the following example, two box plots are added to scatterplot to create an enhanced graph.

```# Add boxplots to a scatterplot par(fig=c(0,0.8,0,0.8), new=TRUE) plot(mtcars\$wt, mtcars\$mpg, xlab="Car Weight",   ylab="Miles Per Gallon") par(fig=c(0,0.8,0.55,1), new=TRUE) boxplot(mtcars\$wt, horizontal=TRUE, axes=FALSE) par(fig=c(0.65,1,0,0.8),new=TRUE) boxplot(mtcars\$mpg, axes=FALSE) mtext("Enhanced Scatterplot", side=3, outer=TRUE, line=-3) ```

To understand this graph, think of the full graph area as going from (0,0) in the lower left corner to (1,1) in the upper right corner. The format of the fig= parameter is a numerical vector of the form c(x1, x2, y1, y2). The first fig= sets up the scatterplot going from 0 to 0.8 on the x axis and 0 to 0.8 on the y axis. The top boxplot goes from 0 to 0.8 on the x axis and 0.55 to 1 on the y axis. I chose 0.55 rather than 0.8 so that the top figure will be pulled closer to the scatter plot. The right hand boxplot goes from 0.65 to 1 on the x axis and 0 to 0.8 on the y axis. Again, I chose a value to pull the right hand boxplot closer to the scatterplot. You have to experiment to get it just right.

fig= starts a new plot, so to add to an existing plot use new=TRUE.

You can use this to combine several plots in any arrangement into one graph.

## To Practice

Try the free first chapter of this interactive data visualization course, which covers combining plots.