type_abline: Add straight lines to a plot

In tinyplot: Lightweight Extension of the Base R Graphics System

Add straight lines to a plot

Description

These functions add straight line(s) through the current plot.

Usage

type_abline(a = 0, b = 1)

type_hline(h = 0)

type_vline(v = 0)

Arguments

a, b	the intercept (default: a = 0) and slope (default: b = 1) terms. Numerics of length 1, or equal to the number of groups or number of facets (or the product thereof).
h	y-value(s) for horizontal line(s). Numeric of length 1, or equal to the number of groups or number of facets (or the product thereof).
v	x-value(s) for vertical line(s). Numeric of length 1, or equal to the number of groups or number of facets (or the product thereof).

Details

While type_abline, type_hline, and type_vline can be called in a base plot layer, we expect that they will typically be called as subsequent layers via tinyplot_add.

Recycling logic

The recycling behaviour of the line parameters (i.e., a, b, h, or v) is adaptive, depending on whether by or facet grouping is detected. While this leads to different recycling scenarios, the underlying code logic follows sensible heuristics designed to match user expectations.

Parameter lengths must equal one of four options:

1. Single value (i.e., length = 1), i.e. simplest case where the same line is applied uniformly across all groups and facets. Uses the default user colour (e.g. "black", or tpar("palette.qualitative")[1] if a theme is set).

2. Number of by groups, i.e. one parameter per group. For example, tinyplot(mpg ~ wt | factor(cyl), data = mtcars, type = type_hline(h = 21:23)) will give three horizontal lines, with colours matching the user's qualitative palette.

3. Number of facet groups, i.e. one parameter per facet panel. For example: tinyplot(mpg ~ wt, facet = ~am, data = mtcars, type = type_hline(h = c(20,30))) would give separate horizontal lines per facet, but both using the same default color.

4. Product of by and facet groups, i.e. one parameter for each unique by-facet combination. Orders over facets first and then, within that, by group. For example: tinyplot(mpg ~ wt | factor(cyl), facet = ~am, data = mtcars, type = type_hline(h = 21:26)) will give six separate lines, with the first three (21:23) coloured by group in the first facet, and second three (24:26) coloured by by group in the second facet.

Alongside these general rules, we also try to accomodate special cases when other aesthetic parameters like lwd or lty are invoked by the user. See Examples.

Examples

#
## abline

tinyplot(x = -10:10, y = rnorm(21) + -10:10, grid = TRUE)
tinyplot_add(type = "abline")
# same as...
# tinyplot_add(type = type_abline(a = 0, b = 1))

# customize by passing bespoke intercept and slope values
tinyplot_add(type = type_abline(a = -1, b = -0.5))

# note that calling as abline & co. as a base plot layer will still lead to
# axes limits that respect the range of the data
tinyplot(x = -10:10, y = -10:10, grid = TRUE, type = "abline")

#
## hline and vline

# Base plot layer
tinyplot(mpg ~ hp | cyl, facet = "by", data = mtcars, ylim = c(0, 40))

# Add horizontal lines at the (default) 0 y-intercept
tinyplot_add(type = "hline", col = "grey")

# Note that group+facet aesthetics will be inherited. We can use this to
# add customized lines (here: the mean `mpg` for each `cyl` group)  
tinyplot_add(type = type_hline(with(mtcars, tapply(mpg, cyl, mean))), lty = 2)

# Similar idea for vline
tinyplot_add(type = type_vline(with(mtcars, tapply(hp, cyl, mean))), lty = 2)

#
## Recycling logic

# length(h) == no. of groups
tinyplot(mpg ~ wt | factor(cyl), data = mtcars, type = type_hline(h = 21:23))

# length(h) == no. of facets
tinyplot(mpg ~ wt, facet = ~am, data = mtcars, type = type_hline(h = c(20, 30)))

# length(h) == no. of groups x no. of facets
tinyplot(mpg ~ wt | factor(cyl), facet = ~am, data = mtcars,
   type = type_hline(h = 21:26))

# special adjustment case (here: lwd by group)
tinyplot(mpg ~ wt | factor(cyl), facet = ~am, data = mtcars,
   type = type_hline(c(20, 30)), lwd = c(21, 14, 7))

tinyplot documentation built on Aug. 8, 2025, 6:23 p.m.