Add NA row for each group in data frame

add-na-rows.r

# Useful for drawing polygons with leaflet
# Polygons are stored in a `tbl_df` object with a mandatory `NA` row between each
# polygon so that `leaflet` knows to stop drawing between each polygon.
# Rather than magic, I found a slick way to do this via `dplyr::arrange`
# See: (http://stackoverflow.com/a/25267681/234233).

# Example using Iris data set:
df_na <- matrix(NA, nrow=nlevels(iris$Species), ncol=ncol(iris) - 1)
df_na <- tbl_df(as.data.frame(df_na))
colnames(df_na) <- setdiff(colnames(iris), "Species")
df_na$Species <- factor(levels(iris$Species))

bind_rows(df_na, iris) %>%
  arrange(!is.na(Species), Species)

# Results look like:
# 
# Source: local data frame [153 x 5]
#
#   Sepal.Length Sepal.Width Petal.Length Petal.Width Species
# 1            NA          NA           NA          NA  setosa
# 2           5.1         3.5          1.4         0.2  setosa
# 3           4.9         3.0          1.4         0.2  setosa
# 4           4.7         3.2          1.3         0.2  setosa
# 5           4.6         3.1          1.5         0.2  setosa
# 6           5.0         3.6          1.4         0.2  setosa
# 7           5.4         3.9          1.7         0.4  setosa
# 8           4.6         3.4          1.4         0.3  setosa
# 9           5.0         3.4          1.5         0.2  setosa
# 10          4.4         2.9          1.4         0.2  setosa
# ..          ...         ...          ...         ...     ...