adapt plotSPC to safely deal with and sketch horizon-less SPC members

[dylanbeaudette]

Examples needed, perhaps in a new example dataset.

[dylanbeaudette]

Seems to work:

library(aqp)

x <- lapply(letters[1:10], random_profile, SPC = FALSE, n_prop = 1)
x <- do.call('rbind', x)

# add an 11th profile, with 1 "slice" that has NA depths
x <- rbind(x, data.frame(id = letters[11], top = NA, bottom = NA, name =NA, p1 = NA))

# promotion to SPC works
depths(x) <- id ~ top + bottom

# plot works
par(mar = c(1, 1, 3, 2))
plotSPC(x, color = 'p1', name = 'name')

# errors as expected
d <- dice(x)

# works
d <- dice(x, fm = 0:50 ~ ., byhz = FALSE)

# note that profile 'k' has been dropped
plotSPC(d, color = 'p1', lwd = 0.5)