R - How to modify the legend?

I'd like to modify the legend in my plot (see below) by adjusting the number of columns (e.g. from one column with nine rows to three columns and three rows). I already tried to add guides(alpha = guide_legend(ncol = 3)) to adjust the number of columns. However, that didn't work (I assume 'alpha' is not the correct argument but I can't find the appropriate one).

Example code and plot:

library(dplyr) #required for tibbles
library(igraph) #required for plotting
library(ggraph) #required for plotting
library(Cairo) #required for anti-aliasing

nodes = LETTERS[1:10] #define nodes
nc = expand.grid(nodes,nodes) #connect all nodes with edges
nc = nc[-which(nc[,1] == nc[,2]),] #remove node connections to themselves 
set.seed(666)
nc = cbind(nc, runif(nrow(nc),0,1)) #add random weights
ne = tibble(from = nc[,1], to = nc[,2], wt = as.numeric(nc[,3])) 

g = graph_from_data_frame(ne) #create graph from nodes and edges
CairoWin() #open window with anti-aliased plot
ggraph(g, layout = "circle") + 
  geom_edge_link(aes(alpha = wt), edge_width = 1) + #add edges with weight
  geom_node_point(size = 10, fill = "white", color = "black", shape = 21, stroke = 1.5) + 
  geom_node_text(aes(label = name)) +
  scale_edge_alpha_continuous(name = "Weight", seq(0,1,0.1)) +
  theme_void()

Help is much appreciated!

Solution

It's because the alpha for edges is internally called edge_alpha and not alpha like the aes() would let you believe. Therefore, you can use the scale_edge_alpha_continuous() to define a legend:

library(dplyr) #required for tibbles
library(igraph) #required for plotting
library(ggraph) #required for plotting

nodes = LETTERS[1:10] #define nodes
nc = expand.grid(nodes,nodes) #connect all nodes with edges
nc = nc[-which(nc[,1] == nc[,2]),] #remove node connections to themselves 
set.seed(666)
nc = cbind(nc, runif(nrow(nc),0,1)) #add random weights
ne = tibble(from = nc[,1], to = nc[,2], wt = as.numeric(nc[,3])) 

g = graph_from_data_frame(ne) #create graph from nodes and edges

p <- ggraph(g, layout = "circle") + 
  geom_edge_link(aes(alpha = wt), edge_width = 1) + #add edges with weight
  geom_node_point(size = 10, fill = "white", color = "black", shape = 21, stroke = 1.5) + 
  geom_node_text(aes(label = name)) +
  theme_void() 

p + scale_edge_alpha_continuous(name = "Weight", seq(0,1,0.1),
                                guide = guide_legend(nrow = 3))

Alternatively, you can also specify it with the guides() function if you name the correct aesthetic.

p + scale_edge_alpha_continuous(name = "Weight", seq(0,1,0.1)) +
  guides(edge_alpha = guide_legend(nrow = 3))

^{Created on 2022-01-21 by the reprex package (v2.0.1)}