Enable manual setting of axis tick labels with coord_sf. Closes #2857

[clauswilke]

This PR closes #2857. It enables most ways in which axis tick labels can normally be set for coord_sf(). The main limitation is that it can only accept character vectors. Parsed expressions will not work. (However, strings that contain the degree symbol will automatically be parsed into expressions.)

library(sf)
#> Linking to GEOS 3.6.1, GDAL 2.1.3, proj.4 4.9.3
library(ggplot2) 

nc <- st_read(system.file("gpkg/nc.gpkg", package="sf"))
#> Reading layer `nc.gpkg' from data source `/Library/Frameworks/R.framework/Versions/3.5/Resources/library/sf/gpkg/nc.gpkg' using driver `GPKG'
#> Simple feature collection with 100 features and 14 fields
#> geometry type:  MULTIPOLYGON
#> dimension:      XY
#> bbox:           xmin: -84.32385 ymin: 33.88199 xmax: -75.45698 ymax: 36.58965
#> epsg (SRID):    4267
#> proj4string:    +proj=longlat +datum=NAD27 +no_defs

# explicitly supply label values
ggplot() + 
  geom_sf(aes(fill = AREA), data=nc) + 
  coord_sf() +
  scale_x_continuous(labels = letters[1:5]) +
  scale_y_continuous(
    breaks = c(34, 34.5, 35.7),
    labels = c("A", "B", "C")
  )

# set to NULL to remove labels
ggplot() + 
  geom_sf(aes(fill = AREA), data=nc) + 
  coord_sf() +
  scale_x_continuous(labels = NULL) +
  scale_y_continuous(
    breaks = c(34, 34.5, 35.7),
    labels = NULL
  )

# format via function
ggplot() + 
  geom_sf(aes(fill = AREA), data=nc) + 
  coord_sf() +
  scale_x_continuous(
    breaks = c(-84, -80, -76),
    labels = function(x) paste0(x, "*degree*East")
  ) +
  scale_y_continuous(
    breaks = c(34, 34.5, 35.7),
    labels = function(x) paste0(x, "*degree*North")
  )

# error if length mismatch
ggplot() + 
  geom_sf(aes(fill = AREA), data=nc) + 
  coord_sf() +
  scale_x_continuous(labels = letters[1:4])
#> Error: Breaks and labels along x direction are different lengths

Created on 2018-08-25 by the reprex package (v0.2.0).

[hadley]

Hmmm, I don’t love the autoparsing, but I’m not sure how to do better and everything else looks good.

[clauswilke]

I have some thoughts on how autoparsing could be avoided / used only on labels generated by st_graticule(), but it is quite involved and I don't want to get into this right now. The fundamental problem is that in the most general case we can end up with character labels for one direction (e.g. N/S) and parsed expressions for the other (e.g. E/W). To handle this appropriately, we'd have to break the graticule into two components, one for each direction, and store them separately. This would have repercussions in several other places in the code.

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