geom_shading_bar: Geom Layer for Drawing Shading Barplot
In plothelper: New Plots Based on 'ggplot2' and Functions to Create Regular Shapes

This function is similar to geom_bar(aes(x, y), stat="identity") except that it draws bars with shading colors. Unlike gg_shading_bar which is a convenient function, this function is used as a ggplot layer. Accepted properties are different from those in geom_multi_raster and gg_shading_bar.

(1) x. It is the same as that in geom_bar.
(2) y. It is the same as that in geom_bar.
(3) raster. It should be a list with 1 or more character vectors of colors. If the list only has 1 vector, all the bars will use the same shading pattern. If you have, for example, 5 bars to draw, then you have to put 5 vectors of colors into a list. If you use a data frame, it must be a data frame made by package tibble, and the column for raster should be a list.
(4) width. It is the same as that in geom_bar.
(5) flip. The default is FALSE. You only need to use TRUE when you use coord_flip. Use outside the aes(...) function.
(6) modify_raster. If it is TRUE (default), colors will be smoothed using the value of smooth. If raster has enough colors, you can set this to FALSE. It is the same as that in gg_shading_bar.
(7) equal_scale. The default is FALSE. When it is TRUE, a bar will use a certain part of the shading colors according to a global scale. It is the same as that in gg_shading_bar.
(8) smooth. The default is 15. The number of shading colors each bar has. The bigger, the better. It is the same as that in gg_shading_bar.
(9) space. The color space that is used. It can be "rgb" (default) or "Lab".
(10) orientation. This parameter mimics the same parameter used in geom_bar, though acts differently. This enables to flip the x axis and y axis without using coord_flip. If it is NA or "x" (default), it supposes x = SOME LABELS and y = SOME VALUES. If it is "y", you must set x = SOME VALUES and y = SOME LABELS. These effects are the same as geom_bar.

NOTE: the function does interpolation as default, so you does not need to use interpolate parameter. And, unlike gg_shading_bar, this function does not draw lines around rectangles.

geom_shading_bar(
  mapping = NULL,
  data = NULL,
  stat = "identity",
  position = "identity",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE,
  width = 0.9,
  flip = FALSE,
  modify_raster = TRUE,
  smooth = 15,
  equal_scale = FALSE,
  space = "rgb",
  orientation = "x",
  ...
)

`mapping`	aes mapping.
`data`	data. It should be a tbl object.
`stat`	stat.
`position`	position. The parameter will not be used here.
`na.rm`	logical, whether to remove NA values.
`show.legend`	This will not be used because the layer does not create any legend.
`inherit.aes`	logical, whether to inherit aes from ggplot().
`width`	see description.
`flip`	see description.
`modify_raster`	see description or `gg_shading_bar`.
`smooth`	see description.
`equal_scale`	see description or `gg_shading_bar`.
`space`	see description.
`orientation`	see description.
`...`	additional parameters.

# Example 1: use vectors.
x=c("b", "a", "c", "d", "e")
y=c(2, 1, 3, 5, 4)
raster=list(c("blue", "red"), c("green", "orange"), 
	c("cyan", "yellow"), c("purple", "orangered"), c("grey", "red"))
ggplot()+
	geom_shading_bar(aes(x=x, y=y, raster=raster), smooth=40)
#
# Example 2: other parameters
x=1: 5
y=c(1, 2, -3, 5, 4)
raster=list(c("blue", "red"))
ggplot()+
	geom_shading_bar(aes(x=x, y=y, raster=raster), 
		smooth=50, width=0.6, equal_scale=TRUE)+
	scale_x_continuous(breaks=1: 5, labels=letters[1: 5])