First, we need to load the dailysales.csv example data file (you can download this file from the code download section of the book's companion website):

sales<-read.csv("dailysales.csv", header=TRUE)

As the file's name suggests, it contains daily sales data of a product. It has two columns: a date column and a sales column that shows the number of units sold.

Here's the code to make your first line graph:

plot(sales$units~as.Date(sales$date,"%d/%m/%y"),
type="l", #Specify type of plot as l for line
main="Unit Sales in the month of January 2010",
xlab="Date",
ylab="Number of units sold",
col="blue")

We first read the data file using the read.csv() function. We passed two arguments to the function: the name of the file we want to read (dailysales.csv in double quotes) and header=TRUE where we specified that the first row contains column headings. We read the contents of the file and saved it in an object called sales with the left arrow notation.

You must have noticed that the plotting code is quite similar to that for producing a scatter plot. The main difference is that this time, we passed the type argument. The type argument tells the plot() function whether you want to plot points, lines, or other symbols. It can take nine different values.

If the type is not specified, R assumes that you want to plot points as it did in the scatter plot example.

The most important part of the example is the way we read the date using the as.Date() function. Reading dates in R is a bit tricky. R doesn't automatically recognize date formats. The as.Date() function takes two arguments: the first is the variable that contains the date values and the second is the format the date values are stored in. In the example, the dates are in the date/month/year or dd/mm/yyyy format, which we specified as %d/%m/%y in the function call. If the date was in the mm/dd/yyyy format, we'd use %m/%d/%y.

The plot and axes titles and line color are set using the same arguments as for a scatter plot.

If you want to plot another line on the same graph, say daily sales data of a second product, you can use the lines() function:

lines(sales$units2~as.Date(sales$date,"%d/%m/%y"),
col="red")

Line graphs and time series charts are covered in depth in Chapter 5, Creating Line Graphs and Time Series Charts.