Using R for Introductory Statistics : John Verzani

Appendix D 362> x = seq(−2, 2, length=200) # 200 points> y = x^2> plot(x,y,type="1")If we did this with length=5 when defining x, the graph would look very clunky.The abline () function is used to draw a single line across the figure.The characteristics of the lines drawn can be varied. The color of the line segments areset with the col= argument. The line width is set with lwd=. Values bigger than 1 widenthe line. The line type is set with lty=. A value of 1 through 6 produces some predefinedtypes. These are shown in Figure D.1. If more control is sought, consult the lty= portionof the help page ?par.The changes can be made each time a line is drawn. Issuing the command par(lty=2)will cause all subsequent lines on the device to be drawn with style 2 (dashed) by default.Adding a region to a graphicThe rect () function will plot rectangles on a figure, as is done with a histogram. Arectangle is specified by four numbers: the x- and y-coordinates of the lower-left cornerand the upper-right corner. More general regions can be drawn using the polygon ()function. This will plot a polygon specified by its x-and y-coordinates. For polygon(),unlike lines (), the first and last points are connected. This creates a figure containing anarea.The col= argument specifies the interior color of the regions; the line (or border) coloris set using border=. An alternative to filling with color is filling with angled lines. Theseare specified by the arguments angle= (default is 45°) and density= (larger valuesproduce more lines).Adding text to a graphicAdding text to a graphic can be done with text () or, in the special case of adding alegend, with legend(). The main arguments for legend () are the position, which can bespecified interactively with locator (); the text to be added with legend=; and any of pch=,col=, and lty= as desired. These are usually vectors of the same length as the number oflegend items we wish to add.The text () function will add labels to a graph, with the option to format the text. Thepositions are specified with (x,y) values (or locator ()). Text is centered at the (x,y) point,although the at= argument allows for adjustments. The text to add is given to the labels=argument.Basic formatting can be done using the f ont= argument. A value of 1 will produce thedefault text, 2 bold text, 3 italic text, and 4 bold-italic text.Math expressions can be printed as well. The full details are in the help page?plotmath. The basic idea is that R expressions are turned into mathematical expressionsand then printed. For example, expression(x==3) will print as “x=3.” (The expression()function makes “expressions” that can subsequently be evaluated.)■ Example D.2: Showing values for pch= and lty= The following commandsproduce Figure D.1, which illustrates the various plot characters and line types.X11(width=5,height=2,pointsize=12)device# new UNIX