gap_smoother <- gapminder %>%
  filter(continent == "Asia") %>%
  group_by(country) %>%
  # Do somewhat rough interpolation for ranking
  # (Otherwise the ranking shifts unpleasantly fast.)
  complete(year = full_seq(year, 1)) %>%
  mutate(gdpPercap = spline(x = year, y = gdpPercap, xout = year)$y) %>%
  group_by(year) %>%
  mutate(rank = min_rank(-gdpPercap) * 1) %>%
  ungroup() %>%

  # Then interpolate further to quarter years for fast number ticking.
  # Interpolate the ranks calculated earlier.
  group_by(country) %>%
  complete(year = full_seq(year, .5)) %>%
  mutate(gdpPercap = spline(x = year, y = gdpPercap, xout = year)$y) %>%
  # "approx" below for linear interpolation. "spline" has a bouncy effect.
  mutate(rank =      approx(x = year, y = rank,      xout = year)$y) %>%
  ungroup()  %>% 
  arrange(country,year)