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Write a ggplot2 map plot as a QGIS project

Source: R/write_qgs.R
write_qgs.Rd

Converts a ggplot2 plot whose layers are drawn from sf objects (or, for ggplot2::geom_point(), ggplot2::geom_path(), ggplot2::geom_line() and ggplot2::geom_polygon(), from plain data frames) into a QGIS project (.qgs) file. The data of each layer is saved as a GeoPackage under <path minus extension>_data/, and the layer is styled after the plot's trained color scale:

Usage

write_qgs(
  plot,
  path,
  use_plot_crs = FALSE,
  gradient_style = c("graduated", "continuous"),
  overwrite = FALSE,
  layer_names = NULL,
  basemap = NULL
)

Arguments

plot

A ggplot object. Each layer must be backed by sf data, or be one of the supported data.frame geoms (see Data frame layers).

path

Path of the .qgs file to write. Tilde paths (e.g. ~/x.qgs) are expanded.

use_plot_crs

If TRUE, the project (map canvas) CRS is the plot's CRS, resolved the way ggplot2::coord_sf() does: its crs argument if specified, otherwise the CRS of the first layer that defines one. If FALSE (the default), the project CRS is EPSG:3857 (Web Mercator). Either way the layers keep the CRS of their data; QGIS reprojects them on the fly.

gradient_style

How a continuous fill/colour scale is rendered:

  • "graduated" (the default): a graduated renderer with 25 equal-interval classes. The gradient is slightly banded, but the legend shows the classes with their value ranges.

  • "continuous": the exact ggplot2 look. The color is interpolated per feature by a data-defined expression on the symbol color (ramp_color(create_ramp(...), ...)). Caveats: QGIS cannot display a color ramp in the legend for a data-defined color, so the legend is a single swatch without any value labels, and the gradient is only discoverable in the layer styling panel behind the data-defined override of the symbol color, not in the renderer dropdown.

Binned scales are unaffected: their bins are exact in a graduated renderer, so there is nothing to trade off. Requesting "continuous" for a layer with a binned scale keeps the bins, with a warning.

overwrite

If FALSE (the default), writing to a path that already exists is an error. Set to TRUE to overwrite it.

layer_names

Names for the layers, used for the GeoPackage files and in the QGIS layer tree: a character vector with one name per layer, bottom-most first. /, \, |, :, *, ?, ", <, > and control characters cannot be used (the name becomes a file name). If NULL (the default), each layer is named after the first of these that applies:

  • the layer's own name (e.g. geom_sf(name = "counties")),

  • the variable its data came from (e.g. nc for geom_sf(data = nc), or for ggplot(nc) when the variable is unambiguous),

  • the geom (e.g. geom_sf),

with a numbered suffix (nc_2) on collision.

basemap

An XYZ tile layer to add below the vector layers, or NULL (the default) for none. Either a predefined key or an arbitrary XYZ URL template (a string containing the {z}, {x} and {y} placeholders, e.g. "https://tile.openstreetmap.org/{z}/{x}/{y}.png"). The predefined keys are:

XYZ tiles are in EPSG:3857; QGIS reprojects them to the project CRS on the fly.

Value

path, invisibly.

Details

  • a continuous fill/colour scale becomes a graduated renderer with fine-grained equal-interval classes (or a continuously interpolated color, see gradient_style),

  • a binned one (e.g. ggplot2::scale_fill_steps()) becomes a graduated renderer with one class per bin, using the scale's exact bin boundaries and colors,

  • a discrete one becomes a categorized renderer,

  • a layer with no fill/colour mapping becomes a single symbol with the color ggplot2 would have used.

Following ggplot2's semantics for polygons, fill is the interior and colour is the border: a colour scale on a polygon layer colors the outlines while the interior keeps the constant fill. Constant outline colors and widths are taken from the plot as well. Mapping both fill and colour on the same layer is not supported.

Only a bare column name is supported for the fill/colour aesthetics; a constant or a computed expression (e.g. aes(fill = AREA * 2)) is an error.

Data frame layers

A geom_point(), geom_path(), geom_line() or geom_polygon() layer drawn from a plain data frame is converted to an sf layer: one point per row, or one linestring/polygon per group (ggplot2's grouping — an explicit group aesthetic or the interaction of the discrete aesthetics; geom_line() orders each line by x like ggplot2 does, and polygon rings are closed). The plot must use ggplot2::coord_sf(), and the x/y values are taken to be coordinates in the panel CRS: coord_sf()'s crs argument if given, otherwise the CRS of the first sf layer (coord_sf(default_crs = ) is not supported). Like fill/colour, the x/y aesthetics must be bare column names, and the layer must use the identity stat and position.

A point layer keeps every column of the data frame as attributes; a line/polygon layer keeps the columns that are constant within every group, one feature per group (a mapped fill/colour column must be constant within each group). fill = NA/colour = NA (including geom_polygon()'s default colour) render as "not drawn" in QGIS. The size, shape, linetype and alpha aesthetics are not carried over; those symbol properties keep the QGIS defaults.

The project opens zoomed to the plot's displayed range (the panel range, including the default expansion and any ggplot2::coord_sf() xlim/ ylim), reprojected to the project CRS, rather than the whole world.

Examples

library(ggplot2)

nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)
p <- ggplot(nc) +
  geom_sf(aes(fill = AREA))

write_qgs(p, tempfile(fileext = ".qgs"))