x<-seq(1,30)

dado<-c(0.96523390,0.93066061,0.89634466,0.86213300,0.82769878,
  0.79311455,0.75831596,0.72311471,0.68800759,0.65245482,0.61700818,
  0.58163643,0.51021060,0.47393336,0.43788203,0.40203755,0.36614804,
  0.33059801,0.29408090,0.25820874,0.22265365,0.18803136,0.15444785,
  0.11931985,0.08411248,0.05098459,0.01957279,0.01358753,0.01257345,
  0.00986485)

plot(x,dado,type="l")

tercil<-quantile(x,probs=1/3)
abline(v=tercil,col="red",lwd=2)

x<-seq(1,30)

dado<-c(0.96523390,0.93066061,0.89634466,0.86213300,0.82769878,
        0.79311455,0.75831596,0.72311471,0.68800759,0.65245482,
        0.61700818,0.58163643,0.51021060,0.47393336,0.43788203,
        0.40203755,0.36614804,0.33059801,0.29408090,0.25820874,
        0.22265365,0.18803136,0.15444785,0.11931985,0.08411248,
        0.05098459,0.01957279,0.01358753,0.01257345,0.00986485)

plot(x,dado,type="l")

tercil<-quantile(x,probs=1/3)

segments(tercil, -1, tercil, dado[ceiling(tercil)], col = "red")
segments(-1, dado[ceiling(tercil)], tercil, dado[ceiling(tercil)], col = "red")

segments(tercil, -1, tercil, dado[ceiling(tercil)]+.01, col = "red")
segments(-1, dado[ceiling(tercil)]+0.01, tercil, dado[ceiling(tercil)]+.01, col = "red")