Sets vpc observation layer
pmx_vpc_obs(show = TRUE, color = "#000000", size = 1, alpha = 0.7,
shape = 1)logical if TRUE show observation points
character Color of the observed endpoint values. Default: "#000000".
numeric Size of the observed endpoint values. Default: 1.
numeric Transparency of the observed endpoint values. Default: 0.7.
numeric Shape of the observed endpoint values. Default: 1.
Other vpc: pmx_plot_vpc,
pmx_vpc_bin, pmx_vpc_ci,
pmx_vpc_pi, pmx_vpc_rug,
pmx_vpc