expect_traces <- function(gg, n.traces, name){
  stopifnot(is.numeric(n.traces))
  L <- expect_doppelganger_built(gg, paste0("smooth-", name))
  all.traces <- L$data
  no.data <- sapply(all.traces, function(tr) {
    is.null(tr[["x"]]) && is.null(tr[["y"]])
  })
  has.data <- all.traces[!no.data]
  expect_equivalent(length(has.data), n.traces)
  list(data = has.data, layout = L$layout)
}

p <- ggplot(mtcars, aes(mpg, wt)) + geom_point() + geom_smooth()

test_that("geom_point() + geom_smooth() produces 3 traces", {
  expect_traces(p, 3, "basic")
})

p2 <- ggplot(mtcars, aes(mpg, wt)) + geom_point() + 
  geom_smooth(se = FALSE)

test_that("geom_point() + geom_smooth(se = FALSE) produces 2 traces", {
  expect_traces(p2, 2, "se-false")
})

d <- diamonds[sample(nrow(diamonds), 1000), ]
p3 <- qplot(carat, price, group = cut, data = d) + geom_smooth()

test_that("geom_smooth() respects group aesthetic", {
  info <- expect_traces(p3, 3, "group")
})

p4 <- qplot(carat, price, colour = cut, data = d) + geom_smooth()
p5 <- qplot(carat, price, data = d) + geom_smooth(aes(colour = cut))

test_that("geom_smooth() respects colour aesthetic", {
  info <- expect_traces(p4, 15, "colour")
  
  # 5 traces of points
  expect_equivalent(
    sum(vapply(info$data, function(x) x$mode == "markers", logical(1))), 5
  )
  # at least 5 paths
  expect_true(
    sum(vapply(info$data, function(x) x$mode == "lines", logical(1))) > 5
  )
})

p7 <- qplot(carat, price, data = d) + geom_smooth(aes(fill = cut))

test_that("geom_smooth() respects fill aesthetic", {
  info <- expect_traces(p7, 11, "fill2")
})

# ensure legend is drawn when needed
p8 <- qplot(carat, price, data = d) + facet_wrap(~cut) + 
  geom_smooth(aes(colour = cut, fill = cut))

test_that("geom_smooth() works with facets", {
  # 3 traces for each panel
  info <- expect_traces(p8, 15, "facet")
})