Biostatistics with R

Multiple curves on the same plot

To plot more than one curve on a single plot in R, we proceed as follows. Create the first plot using the plot() function. For the subsequent plots, do not use the plot() function, which will overwrite the existing plot. Instead, each one of the subsequent curves are plotted using points() and lines() functions, whose calls are similar to the plot().


points() is a generic function to draw a sequence of points at the specified coordinates. The lines() function creates curves by joining a sequence of given points with line segments.


In the R script shown below, we plot the curves of three data sets (x,y1),(x,y2) and (x,y3) on the same plot. The code and the plot are shown here: 




# define 3 data sets
 xdata <- c(1,2,3,4,5,6,7)
 y1 <- c(1,4,9,16,25,36,49)
 y2 <- c(1, 5, 12, 21, 34, 51, 72)
 y3 <- c(1, 6, 14, 28, 47, 73, 106 )

# plot the first curve by calling plot() function
# First curve is plotted
 plot(xdata, y1, type="o", col="blue", pch="o", lty=1, ylim=c(0,110) )

# Add second curve to the same plot by calling points() and lines()
# Use symbol '*' for points.
 points(xdata, y2, col="red", pch="*")
 lines(xdata, y2, col="red",lty=2)

# Add Third curve to the same plot by calling points() and lines()
# Use symbol '+' for points.
 points(xdata, y3, col="dark red",pch="+")
 lines(xdata, y3, col="dark red", lty=3)

Adding legends to the multiple curves

Legends can be added to describe the different curves in the plot. For this, we call the legend() function after plotting the curves. This funciton adds a legend box with appropriate legends at a desired location inside the plot. Some of the important parameters of the legend() function are: 



x, y -----> X and Y axis locations in the graph coordinates.

legend-----> A vector of string consisting of legends, typically one per graph
 
col -----> A vector of colors for 'col' parameter. These colors are same as the ones 
             used in the graph

pch -----> A vector of character symbols for 'pch' parameter, same as the ones 
             used as 'pch' parameters in the plots.

lty -----> A vector of line types to be given to 'lty' parameter, same as the one
             used for plotting curved

ncol -----> Number of columns for writing the legends. Default is one column.

 For other parameters, type  help()   in R prompt.


While typing the vectors for  legend,  col and  pch,  the order of the  plot types
 have to be maintained.

Carefully note the example code below which adds legend to the plot. The resulting figure has three curves with a legend box: 




# define 3 data sets
xdata <- c(1,2,3,4,5,6,7)
y1 <- c(1,4,9,16,25,36,49)
y2 <- c(1, 5, 12, 21, 34, 51, 72)
y3 <- c(1, 6, 14, 28, 47, 73, 106 )

# plot the first curve by calling plot() function
# First curve is plotted
plot(xdata, y1, type="o", col="blue", pch="o", lty=1, ylim=c(0,110), ylab="y" )

# Add second curve to the same plot by calling points() and lines()
# Use symbol '*' for points.
points(xdata, y2, col="red", pch="*")
lines(xdata, y2, col="red",lty=2)

# Add Third curve to the same plot by calling points() and lines()
# Use symbol '+' for points.
points(xdata, y3, col="dark red",pch="+")
lines(xdata, y3, col="dark red", lty=3)

# Adding a legend inside box at the location (2,40) in graph coordinates.
# Note that the order of plots are maintained in the vectors of attributes.
legend(1,100,legend=c("y1","y2","y3"), col=c("blue","red","black"),
                                   pch=c("o","*","+"),lty=c(1,2,3), ncol=1)