The short hand functions in the table’s right column simply are
almost identical to those in the middle column. If one of the side
arguments are specified with a character value, the relevant axis
drawing function is used. You can however choose to use e.g.
brackets_horizontal in place.
The axis drawing functions
** Following section no longer applicable as of ggplot2 v. 3.3.0
(-ish)! **
The functions capped_horizontal or
brackets_vertical returns a function that is called when
ggplot2 prints the plot. In this package, we use ggplot2 to build the
axes, then modify in place the return grobs.
The function is called by the coord objects when
printing the plot, and is called with the arguments
scale_details, axis, scale, position, theme
The function should then return a grob. Some pointers to how it is
used can be found in ggplot2’s help pages on ggproto
(?"ggplot2-ggproto")
The arguments are as follows
scale_details: Details of the scales in ‘npc’ units (see
grid::unit). In the example below, a secondary y-axis is
used, so we find both y.major, etc., and
y.sec.major, etc.
List of 20
$ x.range : num [1:2] 0.4 3.6
$ x.labels : chr [1:3] "a" "b" "c"
$ x.major : num [1:3] 0.187 0.5 0.812
$ x.minor : NULL
$ x.major_source : int [1:3] 1 2 3
$ x.minor_source : NULL
$ x.arrange : chr [1:2] "secondary" "primary"
$ y.range : num [1:2] -2.17 2.26
$ y.labels : chr [1:5] "-2" "-1" "0" "1" ...
$ y.major : num [1:5] 0.0378 0.2635 0.4892 0.7149 0.9406
$ y.minor : num [1:9] 0.0378 0.1506 0.2635 0.3763 0.4892 ...
$ y.major_source : num [1:5] -2 -1 0 1 2
$ y.minor_source : num [1:9] -2 -1.5 -1 -0.5 0 0.5 1 1.5 2
$ y.sec.range : num [1:2] -10.8 11.3
$ y.sec.labels : chr [1:5] "-10" "-5" "0" "5" ...
$ y.sec.major : num [1:5] 0.038 0.263 0.489 0.715 0.941
$ y.sec.minor : num [1:9] 0.0378 0.1506 0.2635 0.3763 0.4892 ...
$ y.sec.major_source: num [1:5] -10 -5 0 5 10
$ y.sec.minor_source: num [1:9] -10 -7.5 -5 -2.5 0 2.5 5 7.5 10
$ y.arrange : chr [1:2] "primary" "secondary"
The secondary y-axis mulitplied the values by 5. Ticks are drawn at
major coordinates, as are major grid lines. Minor grid
lines, if the theme supports them, are drawn at minor.
Observe the connection between y.label and
y.major_source; it only becomes apparent when also
observing the same for the x-axis. The x.major_source are
the factor integers.
axis: Character, either "primary" or
"secondary". Recall that the function is called per
axis drawn.
scale: Character, either "x" or
"y".
position: Character, one of the sides,
"top", "right", "bottom", or
"left".
theme: The plot’s theme. This is however not in absolute
terms, e.g. so text sizes may be described in relative terms to the base
size. To resolve a usable gp (graphic parameters, see
?grid::gpar) for a grob, use
ggplot2:::element_render(theme, 'axis.text.x'), in which
the second argument would resolve to the labels of the x-axis.
Brackets
The brackets comes in two orientations:
brackets_horizontal and brackets_vertical. If
you attempt to use a vertical bracket on a horizontal axis, it will fail
with a undescriptive error.
The bracket functions accept a direction argument, which
can be used to control which direction the end-points are pointing:
p <- ggplot(mpg, aes(cyl, hwy, colour=class)) +
geom_point(position=position_jitter(width=0.3)) +
scale_x_continuous(breaks=c(4,5,6,8), sec.axis=dup_axis()) +
scale_y_continuous(sec.axis=dup_axis()) +
coord_flex_cart(bottom=brackets_horizontal(),
top=brackets_horizontal(direction='down'),
left=brackets_vertical(),
right=brackets_vertical(direction='right')) +
my.theme
p
The look of the brackets are controlled via
theme(axis.ticks). The length of the end-point are
controlled via the theme theme(axis.ticks.length). If these
needs to be specified for each margin, use the argument
tick.length:
p <- ggplot(mpg, aes(cyl, hwy, colour=class)) +
geom_point(position=position_jitter(width=0.3)) +
coord_flex_cart(bottom=brackets_horizontal(tick.length=0),
left=brackets_vertical(tick.length = unit(1, 'cm'))) +
my.theme
p
As shown above, using tick.length=unit(0, 'cm') results
in a flat line.
