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Adding Points, Lines, and Legends to Existing Plots

Once you have created a plot, you can add points, lines, text, or a legend. To place each of these elements, R uses coordinates defined in terms of the x-axes and y-axes of the plot area, not coordinates defined in terms of the the plotting window or device. For example, if your plot has an x-axis with values between 33#33 , and a y-axis with values between 34#34 , you may add a point at 35#35 .

• points() plots one or more sets of points. Use pch with points to add points to an existing plot. For example, points(P, Q, pch = ".", col = "forest green") plots each 36#36 as tiny green dots.
• lines() joins the specified points with line segments. The arguments col and lty may also be used. For example, lines(X, Y, col = "blue", lty = "dotted") draws a blue dotted line from each set of points 29#29 to the next. Alternatively, lines also takes command output which specifies 37#37 coordinates. For example, density(Z) creates a vector of 30#30 and a vector of 31#31 , and plot(density(Z)) draws the kernel density function.
• text() adds a character string at the specified set of 37#37 coordinates. For example, text(5, 5, labels = "Key Point") adds the label ``Key Point'' at the plot location 38#38 . You may also choose the font using the font option, the size of the font relative to the axis labels using the cex option, and choose a color using the col option. The full list of options may be accessed using help(text).
• legend() places a legend at a specified set of 37#37 coordinates. Type demo(vertci) to see an example for legend().