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