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Plotting pies on ggplot/ggmap is not an easy task, as ggplot2 doesn’t provide native pie geom. The pie we produced in ggplot2 is actually a barplot transform to polar coordination. This make it difficult if we want to produce a map like the above screenshot, which was posted by Tyler Rinker, the author of R package pacman.

The question remained unsolved until he discovered that ggtree can do it. The ggtree solution is to use the `subview`

function, which is good for embeding subplots and can embed different types of plots and even user’s own image files.

But it has its own drawbacks for plotting pies on map. First, it render plots as raster image make it slow to render when we plotting a lot of pies. Second we need some hacks to add legend.

Thanks to the `ggforce`

package, which provides a native implementation of the pie geom, we can plot pies on cartesian coordination.

I created a wrapper function to make it more easy to plot a set of pies.

For example, suppose we have the following data:

```
set.seed(123)
long <- rnorm(50, sd=100)
lat <- rnorm(50, sd=50)
d <- data.frame(long=long, lat=lat)
d <- with(d, d[abs(long) < 150 & abs(lat) < 70,])
n <- nrow(d)
d$region <- factor(1:n)
d$A <- abs(rnorm(n, sd=1))
d$B <- abs(rnorm(n, sd=2))
d$C <- abs(rnorm(n, sd=3))
d$D <- abs(rnorm(n, sd=4))
d$radius <- 6 * abs(rnorm(n))
head(d)
## long lat region A B C D
## 1 -56.047565 12.665926 1 0.71040656 2.887786 1.309570 2.892264
## 2 -23.017749 -1.427338 2 0.25688371 1.403569 1.375096 4.945092
## 4 7.050839 68.430114 3 0.24669188 0.524395 3.189978 5.138863
## 5 12.928774 -11.288549 4 0.34754260 3.144288 3.789556 2.295894
## 8 -126.506123 29.230687 5 0.95161857 3.029335 1.048951 2.471943
## 9 -68.685285 6.192712 6 0.04502772 3.203072 2.596539 4.439393
## radius
## 1 6.4847970
## 2 3.7845247
## 4 0.6818394
## 5 9.1974120
## 8 3.1267039
## 9 2.9392227
```

It is very easy to draw the pies on the map by the `geom_scatterpie`

layer.

```
library(ggplot2)
library(scatterpie)
world <- map_data('world')
p <- ggplot(world, aes(long, lat)) +
geom_map(map=world, aes(map_id=region), fill=NA, color="black") +
coord_quickmap()
p + geom_scatterpie(aes(x=long, y=lat, group=region, r=radius),
data=d, cols=LETTERS[1:4], color=NA, alpha=.8) +
geom_scatterpie_legend(d$radius, x=-160, y=-55)
```

This is just a simple application of the `ggforce`

, and I find many people like it. They even asked me to implement the pie size legend. I do implemented a `geom_scatterpie_legend`

layer and as the name indicated, it add a legend of the pie sizes as demonstrated in the above figure.

The source code is quite simple, and it is impossible without `ggforce`

. Now this package is availabel on CRAN, you can use `install.packages('scatterpie')`

to install it and visit the online vignette.

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