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I'm new to ggplot2 and relatively new to R. I can make a picture appear on a plot, and I can make the y axis reverse scale, but I don't know how to do both at once. For example:
library(ggplot2)
y=c(1,2,3)
x=c(0,0,0)
d=data.frame(x=x, y=y)
#following http://stackoverflow.com/questions/9917049/inserting-an-image-to-ggplot2/9917684#9917684
library(png)
library(grid)
img <- readPNG(system.file("img", "Rlogo.png", package="png"))
g <- rasterGrob(img, interpolate=TRUE)
#these work fine - either reversing scale, or adding custom annotation
ggplot(d, aes(x, y)) + geom_point()
ggplot(d, aes(x, y)) + geom_point() + scale_y_reverse()
ggplot(d, aes(x, y)) + geom_point() + annotation_custom(g, xmin=.23, xmax=.27, ymin=1.8, ymax=2.2)
#these don't...combining both reverse scale and custom annotation
ggplot(d, aes(x, y)) + geom_point() + annotation_custom(g, xmin=.23, xmax=.27, ymin=1.8, ymax=2.2) + scale_y_reverse()
ggplot(d, aes(x, y)) + geom_point() + annotation_custom(g, xmin=.23, xmax=.27, ymin=2.2, ymax=1.8) + scale_y_reverse()
I'm sure I'm missing something pretty basic. Where should I start looking both to get my little graphic to display on a reverse scale plot, and also to understand things better under the hood?
CLARIFICATION IN RESPONSE TO COMMENTS: The example above is me trying to simplify the problem I'm having. I don't know if it matters, but I'm not merely trying to overlay some data on a static image. I actually want to place an image in a certain spot on a plot, based on the data in the plot. However, I can't seem to do that when the axis scale is reversed. And, as it turns out, I can't even put an image in an absolute position when the scale is reversed either, so that's the code example I posted.
365
First off a grob is just short for “grid graphical object” from the low-level graphics package grid; Think of it as a set of instructions for create a graphical object (i.e. a plot). The graphics library underneath all of ggplot2's graphical elements are really composed of grob's because ggplot2 uses grid underneath.
In ggplot2 syntax, we say that they use different geoms. A geom is the geometrical object that a plot uses to represent data. People often describe plots by the type of geom that the plot uses. For example, bar charts use bar geoms, line charts use line geoms, boxplots use boxplot geoms, and so on.
Geoms. A layer combines data, aesthetic mapping, a geom (geometric object), a stat (statistical transformation), and a position adjustment. Typically, you will create layers using a geom_ function, overriding the default position and stat if needed.
With scale_y_reverse, you need to set the y coordinates inside annotation_custom to their negative.
library(ggplot2)
y=c(1,2,3)
x=c(0,0,0)
d=data.frame(x=x, y=y)
library(png)
library(grid)
img <- readPNG(system.file("img", "Rlogo.png", package="png"))
g <- rasterGrob(img, interpolate=TRUE)
ggplot(d, aes(x, y)) + geom_point() +
annotation_custom(g, xmin=.20, xmax=.30, ymin=-2.2, ymax=-1.7) +
scale_y_reverse()
Why negative? The y coordinates are the negative of the original. Check out this:
(p = ggplot(d, aes(x=x, y=y)) + geom_point() + scale_y_reverse())
y.axis.limits = ggplot_build(p)$layout$panel_params[[1]][["y.range"]]
y.axis.limits
OR, set the coordinates and size of the grob in relative units inside rasterGrob.
g <- rasterGrob(img, x = .75, y = .5, height = .1, width = .2, interpolate=TRUE)
ggplot(d, aes(x, y)) + geom_point() +
annotation_custom(g) +
scale_y_reverse()
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