How to layer a geom behind a ggparcoord? Geom_rect/geom_ribbon keeps blocking the parallel plot

Question

I'm drawing a Parallel coordinates plot using the ggparcoord function from GGAlly. So far it's working fine but now I want to add color to a portion of the plot's background only. I've found solutions online using geom_rect or geom_ribbon, however, those answers were used in a ggplot instead of a ggparcoord.

ggparcoord(data = dat, columns = c(43,61), groupColumn = 74, scale = "globalminmax", alphaLines = 1,showPoints = TRUE) +
  geom_rect(aes(
    xmin="LVEF trước xuất viện",
    xmax="LVEF sau xuất viện 3 tháng",
    ymin=0, ymax=40), inherit.aes=FALSE) +
  scale_color_manual(values=c( "#69b3a2", "#E8E8E8", "#E8E8E8") ) +
  theme_minimal() +
  xlab("") + ylab("%") +
  scale_y_continuous(limits = c(0, 100), breaks = c(0, 20,40,50,70)) +
  scale_x_discrete(expand = c(0.05,0.05))

By trying the same code used in those answers, the color of the ribbon or rectangle keeps blocking the Parallel plot (see attached image). Please help me work around this.

Adding a reproducible example as suggested by MrFlick:

data(diamonds, package="ggplot2")
diamonds.samp <- diamonds[sample(1:dim(diamonds)[1], 100), ]

ggparcoord(data = diamonds.samp, columns = c(1, 5:10)) +
  geom_rect(aes(xmin=-Inf, xmax=Inf,ymin=-2, ymax=0), alpha =0.5) +
  theme_minimal()

As you can see, even with the alpha = 0.5, the rectangle produced by geom_rect still keeps blocking the original plot.

Answer 1

I've found a solution using the annotate function.

annotate("rect", fill="red", alpha =0.1, xmin=-Inf, xmax=Inf, ymin=0, ymax=40)

Hope this helps anyone in need.