How to specify order of glyph layers in rbokeh?

Question

I am trying to build a network visualisation using the rbokeh package.

library(igraph)
library(rbokeh)
library(dplyr)

 g <- random.graph.game(n=100,p=0.3)

 L <- as.data.frame(igraph::layout_with_fr(g)) %>% rename(x=V1,y=V2)

 url1 <- 'http://icons.veryicon.com/png/Business/Flat%20Finance/person.png'

 p <- figure(xlab = "x", ylab = "y", height = 500,width=1000,xgrid=F,ygrid=F,webgl = T,
             xaxes = F,yaxes = F,h_symmetry = T,v_symmetry = T) %>%
   ly_lines(x = L$x,y=L$y,color = '#FFA700', width = 4, alpha = 0.2) %>%
   ly_image_url(x = L$x, y=L$y, image_url = url1, w = rep(0.1,vcount(g)), h=rep(0.2,vcount(g)),
                anchor = "center",lname = 'nodes')

The resulting visualisation looks as intended except for the fact that the lines are drawn on top of the image glyphs. Is there a way to control the visual order of the layers in a way that the nodes (images) are drawn on top with lines drawn behind?

Answer 1

The issue here is using webgl = TRUE. For more details, see the Bokeh documentation (see both the "support" and "notes" sections. The main points there are that not all glyphs can be rendered with WebGL (yes for lines, no for images) and that "Glyphs drawn using WebGL are drawn on top of glyphs that are not drawn in WebGL."

If you get rid of webgl = TRUE, you should be in good shape!

Also, to further answer the overall question of how to control the order of layers, the answer is that outside of edge cases like this where one layer was rendered with WebGL and the other wasn't, layers are drawn in the order that they are specified.