[R] passing xlim to coord_map in ggplot2

[z2.0]

I'm sure this is smack-head moment, but I haven't been able to find an
example of this on Nabble or SO, so thought I'd ask.

This works:
michigan <- map_data('county', 'michigan')
mich_points <- data.frame(x = rnorm(n = 200, median(michigan[,1]), 0.75), y
= rnorm(n = 200, median(michigan[,2]), 0.75))
ggplot() + geom_path(aes(long, lat, group = group), data = michigan) +
geom_point(aes(x, y), data = mich_points) + coord_map('gilbert', xlim =
c(-86, -84))

This generates the following error:
*Error in unit(x, default.units) : 'x' and 'units' must have length > 0*
#Where tank_trunc is a data.frame with two columns, 'lon' and 'lat'
containing point coordinates in storage mode 'double'.
michigan_map.df <- map_data('county', 'michigan')
ggplot() + geom_point(aes(lon, lat), data = tank_trunc, na.rm = T) +
geom_path(aes(long, lat, group = group), data = michigan_map.df) +
coord_map('gilbert', xlim = c(-88, -82))

I thought at first maybe the overlay of one layer on another caused the
limiting to freak out. But the sketch code above disproves that theory --
thoughts? Some kink in ggplot2's latest implementation? Do I need another
package? (e.g., the Scales disunion in the latest release...)

Thanks, as always,

Zack

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