geom_arrow: Arrows
In metR: Tools for Easier Analysis of Meteorological Fields

geom_arrow

R Documentation

Arrows

Description

Parametrization of ggplot2::geom_segment either by location and displacement or by magnitude and angle with default arrows. geom_arrow() is the same as geom_vector() but defaults to preserving the direction under coordinate transformation and different plot ratios.

Usage

geom_arrow(
  mapping = NULL,
  data = NULL,
  stat = "arrow",
  position = "identity",
  ...,
  start = 0,
  direction = c("ccw", "cw"),
  pivot = 0.5,
  preserve.dir = TRUE,
  min.mag = 0,
  skip = 0,
  skip.x = skip,
  skip.y = skip,
  arrow.angle = 15,
  arrow.length = 0.5,
  arrow.ends = "last",
  arrow.type = "closed",
  arrow = grid::arrow(arrow.angle, grid::unit(arrow.length, "lines"), ends = arrow.ends,
    type = arrow.type),
  lineend = "butt",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE
)

geom_vector(
  mapping = NULL,
  data = NULL,
  stat = "arrow",
  position = "identity",
  ...,
  start = 0,
  direction = c("ccw", "cw"),
  pivot = 0.5,
  preserve.dir = FALSE,
  min.mag = 0,
  skip = 0,
  skip.x = skip,
  skip.y = skip,
  arrow.angle = 15,
  arrow.length = 0.5,
  arrow.ends = "last",
  arrow.type = "closed",
  arrow = grid::arrow(arrow.angle, grid::unit(arrow.length, "lines"), ends = arrow.ends,
    type = arrow.type),
  lineend = "butt",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE
)

Arguments

`mapping`	Set of aesthetic mappings created by `aes()`. If specified and `inherit.aes = TRUE` (the default), it is combined with the default mapping at the top level of the plot. You must supply `mapping` if there is no plot mapping.
`data`	The data to be displayed in this layer. There are three options: If `NULL`, the default, the data is inherited from the plot data as specified in the call to `ggplot()`. A `data.frame`, or other object, will override the plot data. All objects will be fortified to produce a data frame. See `fortify()` for which variables will be created. A `function` will be called with a single argument, the plot data. The return value must be a `data.frame`, and will be used as the layer data. A `function` can be created from a `formula` (e.g. `~ head(.x, 10)`).
`stat`	The statistical transformation to use on the data for this layer. When using a `⁠geom_*()⁠` function to construct a layer, the `stat` argument can be used the override the default coupling between geoms and stats. The `stat` argument accepts the following: A `Stat` ggproto subclass, for example `StatCount`. A string naming the stat. To give the stat as a string, strip the function name of the `stat_` prefix. For example, to use `stat_count()`, give the stat as `"count"`. For more information and other ways to specify the stat, see the layer stat documentation.
`position`	A position adjustment to use on the data for this layer. This can be used in various ways, including to prevent overplotting and improving the display. The `position` argument accepts the following: The result of calling a position function, such as `position_jitter()`. This method allows for passing extra arguments to the position. A string naming the position adjustment. To give the position as a string, strip the function name of the `position_` prefix. For example, to use `position_jitter()`, give the position as `"jitter"`. For more information and other ways to specify the position, see the layer position documentation.
`...`	Other arguments passed on to `layer()`'s `params` argument. These arguments broadly fall into one of 4 categories below. Notably, further arguments to the `position` argument, or aesthetics that are required can not be passed through `...`. Unknown arguments that are not part of the 4 categories below are ignored. Static aesthetics that are not mapped to a scale, but are at a fixed value and apply to the layer as a whole. For example, `colour = "red"` or `linewidth = 3`. The geom's documentation has an Aesthetics section that lists the available options. The 'required' aesthetics cannot be passed on to the `params`. Please note that while passing unmapped aesthetics as vectors is technically possible, the order and required length is not guaranteed to be parallel to the input data. When constructing a layer using a `⁠stat_()⁠` function, the `...` argument can be used to pass on parameters to the `geom` part of the layer. An example of this is `stat_density(geom = "area", outline.type = "both")`. The geom's documentation lists which parameters it can accept. Inversely, when constructing a layer using a `⁠geom_()⁠` function, the `...` argument can be used to pass on parameters to the `stat` part of the layer. An example of this is `geom_area(stat = "density", adjust = 0.5)`. The stat's documentation lists which parameters it can accept. The `key_glyph` argument of `layer()` may also be passed on through `...`. This can be one of the functions described as key glyphs, to change the display of the layer in the legend.
`start`	starting angle for rotation in degrees
`direction`	direction of rotation (counter-clockwise or clockwise)
`pivot`	numeric indicating where to pivot the arrow where 0 means at the beginning and 1 means at the end.
`preserve.dir`	logical indicating whether to preserve direction or not
`min.mag`	minimum magnitude for plotting vectors
`skip`, `skip.x`, `skip.y`	numeric specifying number of gridpoints not to draw in the x and y direction
`arrow.length`, `arrow.angle`, `arrow.ends`, `arrow.type`	parameters passed to grid::arrow
`arrow`	specification for arrow heads, as created by `grid::arrow()`.
`lineend`	Line end style (round, butt, square).
`na.rm`	If `FALSE`, the default, missing values are removed with a warning. If `TRUE`, missing values are silently removed.
`show.legend`	logical. Should this layer be included in the legends? `NA`, the default, includes if any aesthetics are mapped. `FALSE` never includes, and `TRUE` always includes. It can also be a named logical vector to finely select the aesthetics to display.
`inherit.aes`	If `FALSE`, overrides the default aesthetics, rather than combining with them. This is most useful for helper functions that define both data and aesthetics and shouldn't inherit behaviour from the default plot specification, e.g. `borders()`.

Details

Direction and start allows to work with different standards. For the meteorological standard, for example, use star = -90 and direction = "cw".

Aesthetics

geom_vector understands the following aesthetics (required aesthetics are in bold)

x
y
either mag and angle, or dx and dy
alpha
colour
linetype
size
lineend

Examples

library(data.table)
library(ggplot2)

data(seals)
# If the velocity components are in the same units as the axis,
# geom_vector() (or geom_arrow(preserve.dir = TRUE)) might be a better option
ggplot(seals, aes(long, lat)) +
    geom_arrow(aes(dx = delta_long, dy = delta_lat), skip = 1, color = "red") +
    geom_vector(aes(dx = delta_long, dy = delta_lat), skip = 1) +
    scale_mag()

data(geopotential)
geopotential <- copy(geopotential)[date == date[1]]
geopotential[, gh.z := Anomaly(gh), by = .(lat)]
geopotential[, c("u", "v") := GeostrophicWind(gh.z, lon, lat)]

(g <- ggplot(geopotential, aes(lon, lat)) +
    geom_arrow(aes(dx = dlon(u, lat), dy = dlat(v)), skip.x = 3, skip.y = 2,
               color = "red") +
    geom_vector(aes(dx = dlon(u, lat), dy = dlat(v)), skip.x = 3, skip.y = 2) +
    scale_mag( guide = "none"))

# A dramatic illustration of the difference between arrow and vector
g + coord_polar()

# When plotting winds in a lat-lon grid, a good way to have both
# the correct direction and an interpretable magnitude is to define
# the angle by the longitud and latitude displacement and the magnitude
# by the wind velocity. That way arrows are always parallel to streamlines
# and their magnitude are in the correct units.
ggplot(geopotential, aes(lon, lat)) +
    geom_contour(aes(z = gh.z)) +
    geom_vector(aes(angle = atan2(dlat(v), dlon(u, lat))*180/pi,
                   mag = Mag(v, u)), skip = 1, pivot = 0.5) +
    scale_mag()

# Sverdrup transport
library(data.table)
b <- 10
d <- 10
grid <- as.data.table(expand.grid(x = seq(1, d, by = 0.5),
                                  y = seq(1, b, by = 0.5)))
grid[, My := -sin(pi*y/b)*pi/b]
grid[, Mx := -pi^2/b^2*cos(pi*y/b)*(d - x)]

ggplot(grid, aes(x, y)) +
    geom_arrow(aes(dx = Mx, dy = My))

# Due to limitations in ggplot2 (see: https://github.com/tidyverse/ggplot2/issues/4291),
# if you define the vector with the dx and dy aesthetics, you need
# to explicitly add scale_mag() in order to show the arrow legend.

ggplot(grid, aes(x, y)) +
    geom_arrow(aes(dx = Mx, dy = My)) +
    scale_mag()

# Alternative, use Mag and Angle.
ggplot(grid, aes(x, y)) +
    geom_arrow(aes(mag = Mag(Mx, My), angle = Angle(Mx, My)))

metR documentation built on Oct. 14, 2024, 5:09 p.m.