Tile layout improvement: makes parts contiguous

[edwindj]

Thanks for this package!

I ran into a layout issue with a data set of mine which is shown in the following example:

parts <- tapply(iris$Sepal.Length, iris$Species, mean)
waffle(parts, rows = 7)

which results in:

I provided a fix by sorting the even rows of the expanded grid in the opposite direction of the odd rows. ("snake_sort"?), resulting in contiguous parts:

I suppose this is related to issue #32.