element_path: Theme Elements for Image Grobs

Description

In conjunction with the ggplot2::theme() system, the element_ functions specify the display of how non-data components of a ggplot are drawn. Both functions call magick::image_read() to process image files from valid image URLs, local paths, raster objects, or bitmap arrays.

element_path(): draws images as replacement for ggplot2::element_text(). Use this to replace text with images.
element_raster(): draws images as replacement for ggplot2::element_rect(). Use this to put images in plot background.

Usage

element_path(
  alpha = NULL,
  colour = NA,
  hjust = NULL,
  vjust = NULL,
  color = NULL,
  angle = NULL,
  size = 0.5
)
element_raster(
  image_path,
  x = grid::unit(0.5, "npc"),
  y = grid::unit(0.5, "npc"),
  width = grid::unit(1, "npc"),
  height = grid::unit(1, "npc"),
  just = "centre",
  hjust = NULL,
  vjust = NULL,
  interpolate = TRUE,
  default.units = "npc",
  name = NULL,
  gp = NULL,
  vp = NULL
)

Value

An S3 object of class element.

Arguments

alpha: The alpha channel, i.e. transparency level, as a numerical value between 0 and 1.
colour, color: The image will be colorized with this color. Use the special character "b/w" to set it to black and white. For more information on valid color names in ggplot2 see https://ggplot2.tidyverse.org/articles/ggplot2-specs.html?q=colour#colour-and-fill.
hjust: A numeric vector specifying horizontal justification. If specified, overrides the just setting.
vjust: A numeric vector specifying vertical justification. If specified, overrides the just setting.
angle: The angle of the element as a numerical value between 0° and 360°.
size: The output grob size in cm (!).
image_path: A file path, url, raster object or bitmap array. See magick::image_read() for further information.
x: A numeric vector or unit object specifying x-location.
y: A numeric vector or unit object specifying y-location.
width: A numeric vector or unit object specifying width.
height: A numeric vector or unit object specifying height.
just: The justification of the rectangle relative to its (x, y) location. If there are two values, the first value specifies horizontal justification and the second value specifies vertical justification. Possible string values are: "left", "right", "centre", "center", "bottom", and "top". For numeric values, 0 means left alignment and 1 means right alignment.
interpolate: A logical value indicating whether to linearly interpolate the image (the alternative is to use nearest-neighbour interpolation, which gives a more blocky result).
default.units: A string indicating the default units to use if x, y, width, or height are only given as numeric vectors.
name: A character identifier.
gp: An object of class "gpar", typically the output from a call to the function gpar. This is basically a list of graphical parameter settings.
vp: A Grid viewport object (or NULL).

Details

To be able to use the functions correctly, a basic understanding of how they work is required.

element_path() can be applied wherever ggplot2::element_text() is usually used. It replaces text with an image if the text is a valid image file location or data.

element_raster() can be applied wherever ggplot2::element_rect() is usually used. A path in the sense of magick::image_read() must be explicitly specified here because it cannot read plot data. It is designed exclusively for inserting an image into the background of a plot and calls grid::rasterGrob() internally. Neither width nor height need to be specified, in which case, the aspect ratio of the image is preserved. If both width and height are specified, it is likely that the image will be distorted.

Examples

Run this code

library(ggplot2)
library(ggpath)

# compute paths of R logo file and background image file shipped with ggpath
local_r_logo <- system.file("r_logo.png", package = "ggpath")
local_background_image <- system.file("example_bg.jpg", package = "ggpath")

# create dataframe with x-y-coordinates and the above local path
plot_data <- data.frame(x = c(-1, 1), y = 1, path = local_r_logo)

# Replace title, subtitle, the caption, axis labels as well as y-axis text
# the the local image
ggplot(plot_data, aes(x = x, y = local_r_logo)) +
  theme_minimal() +
  labs(
    title = local_r_logo,
    subtitle = local_r_logo,
    x = local_r_logo,
    y = local_r_logo,
    caption = local_r_logo
  ) +
  theme(
    plot.caption = element_path(hjust = 1, size = 0.6),
    axis.text.y = element_path(size = 1),
    axis.title.x = element_path(),
    axis.title.y = element_path(vjust = 0.9),
    plot.title = element_path(hjust = 0, size = 2, alpha = 0.5),
    plot.subtitle = element_path(hjust = 0.9, angle = 45),
  )

# Use local image as plot background
ggplot(plot_data, aes(x = x, y = y)) +
  geom_from_path(aes(path = path), width = 0.2) +
  coord_cartesian(xlim = c(-2, 2)) +
  theme_dark() +
  theme(
    plot.background = element_raster(local_background_image),
    panel.background = element_rect(fill = "transparent")
  )