The function was altered from ctmm:::plot.variogram()

plot_variogram(
  x,
  CTMM = NULL,
  level = 0.95,
  units = TRUE,
  fraction = 0.5,
  col = "black",
  col.CTMM = "red",
  xlim = NULL,
  ylim = NULL,
  ext = NULL,
  ...
)

Details

It now returns the internally computed SVF (by using plot_svf - the altered version of ctmm:::plot.svf())