Drawing Line Segments, Arrows, and Boxes

Well-chosen annotations can make a good plot better. In the previous chapter, we saw how labels can be placed anywhere on a plot, using either data or window coordinates. plt also allows you to draw arbitrary line segments, arrows, and boxes on your plot.

Use the -C and -c (connect) options to draw arbitrary line segments, as follows:

-C xw0 yw0 xw1 yw1
-c x0 y0 x1 y1

These options connect the two points specified by their arguments with a line segment. The -C option connects the points with window coordinates $(xw0,yw0)$ and $(xw1,yw1)$. The -c option connects the points with data coordinates $(x0,y0)$ and $(x1,y1)$.

The -A and -a (arrow) options work in the same way as -C and -c, respectively, but they also draw an arrowhead pointing toward the first specified point, $(xw0,yw0)$ or $(x0,y0)$:

-A xw0 yw0 xw1 yw1
-a x0 y0 x1 y1

Note that the size of the arrowhead is determined by the specified point size in the figure (f) font group, or in local override instructions (see figure 11.4).

The -B and -b (box) options similarly accept arguments specifying two points, but in this case the points are diagonally opposite corners of a box (rectangle) that is to be drawn:

-B xw0 yw0 xw1 yw1
-b x0 y0 x1 y1

The -D and -d (dark box) options work in the same way as -B and -b respectively, but boxes drawn when using these options are filled. The filling is black by default, but may be changed by setting the grey level or color in the f (figure) font group (see figure 11.5).

-D xw0 yw0 xw1 yw1
-d x0 y0 x1 y1

Remember that in each case, as with -L and -l, the upper case version of the option takes window coordinates, and the lower case version takes data coordinates.

Figure 9.1: Line segments, arrows, and boxes

These options are illustrated in figure 9.1, which was produced using the command:

plt -f flowchart.format

where the format file, flowchart.format, contains:

# Define axis ranges, but don't draw the axes.
X 0 1
Y 0 1
s xy
# Use black, wide lines for figures by default.
sf f Cblack,W15,P20
# Draw labels in black 20 point Helvetica Bold.
sf l Cblack,P20,Fh-b
# Draw the plot title in 24 point Helvetica Bold.
t (Fh-b,P24) How to Hack
# Draw a green-outlined box and label it.
B (Cgreen) .1 .3 .3 .4
L .2 .35 CC Write
# Draw an arrow from the right edge of the box.
A .4 .35 .3 .35
# Draw a diamond using line segments, and label it.
C (Cred) .4 .35 .5 .45
C (Cred) .5 .45 .6 .35
C (Cred) .6 .35 .5 .25
C (Cred) .5 .25 .4 .35
L .5 .35 CC Done?
# Draw a line and arrow from the top of the diamond.
C .5 .45 .5 .7
A .4 .7 .5 .7
# Label the line and arrow.
L .51 .45 LB No
# Draw a filled light blue box.
D (CLightBlue) .2 .65 .4 .75
# Outline it in dark blue, and label it.
B (CDarkBlue) .2 .65 .4 .75
L .3 .7 CC Think!
# Draw a line and arrow from this box to the first.
C .2 .7 .15 .7
A .15 .4 .15 .7
# Draw an arrow from the right corner of the diamond.
A .7 .35 .6 .35
# Label the arrow.
L .6 .35 LT Yes
# Draw a filled box in 30% grey, and label it in white.
D (Cgrey30) .7 .3 .9 .4
L (Cwhite) .8 .35 CC Sleep

The -s option, used in this example to suppress the axes, is described in the next chapter. The -sf option, used to set the default characteristics for the figures and labels in the example, is discussed in chapter 11, beginning on page .