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Drawing Plots

The most generic plotting command is plot(), which automatically recognizes the type of R object(s) you are trying to plot and selects the best type of plot. The most common graphs returned by plot() are as follows:

1. If X is a variable of length $n$, plot(X) returns a scatter plot of $(x_i, i)$ for $i = 1, \dots n$. If X is unsorted, this procedure produces a messy graph. Use plot(sort(X)) to arrange the plotted values of $(x_i, i)$ from smallest to largest.
2. With two numeric vectors X and Y, both of length $n$, plot(X, Y) plots a scatter plot of each point $(x_i, y_i)$ for $i = 1, \dots n$. Alternatively, if Z is an object with two vectors, plot(Z) also creates a scatter plot.

Optional arguments specific to plot include:

• main creates a title for the graph, and xlab and ylab label the x and y axes, respectively. For example,

  plot(x, y, main = "My Lovely Plot", xlab = "Explanatory Variable", 
       ylab = "Dependent Variable")
  

• type controls the type of plot you request. The default is plot(x, y, type = "p"), but you may choose among the following types:
  

  "p"	points
  "l"	lines
  "b"	both points and lines
  "c"	lines drawn up to but not including the points
  "h"	histogram
  "s"	a step function
  "n"	a blank plotting region ( with the axes specified)

  
  
• If you choose type = "p", R plots open circles by default. You can change the type of point by specifying the pch argument. For example, plot(x, y, type = "p", pch = 19) creates a scatter-plot of filled circles. Other options for pch include:
  

  19	solid circle (a disk)
  20	smaller solid circle
  21	circle
  22	square
  23	diamond
  24	triangle pointed up
  25	triangle pointed down

  
  

  In addition, you can specify your own symbols by using, for example, pch = "*" or pch = ".".

• If you choose type = "l", R plots solid lines by default. Use the optional lty argument to set the line type. For example, plot(x, y, type = "l", lty = "dashed") plots a dashed line. Other options are dotted, dotdash, longdash, and twodash.

• col sets the color of the points, lines, or bars. For example, plot(x, y, type = "b", pch = 20, lty = "dotted", col = "violet") plots small circles connected by a dotted line, both of which are violet. (The axes and labels remain black.) Use colors() to see the full list of available colors.

• xlim and ylim set the limits to the $x$-axis and $y$-axis. For example, plot(x, y, xlim = c(0, 25), ylim = c(-15, 5)) sets range of the $x$-axis to [0, 25] and the range of the $y$-axis to $[-15, 5]$.

For additional plotting options, refer to help(par).