如何避免igraph绘制有向图时出现双向边箭头重叠问题

双向边重叠是igraph默认使用直边绘制导致的，互反的两条边会完全重合，仅显示为双箭头。可以通过调整边的曲率参数实现双向边分离显示。

解决方案1：全局统一设置边曲率 #

最简单的方案是在plot()函数中添加edge.curved参数，设置0.1~0.3的曲率值即可让双向边自然错开：

library(igraph)

actors <- data.frame(name=c("a","b","c","d","e","f","g","h"))
relations <- data.frame(from=c("a", "a", "b", "b",
                               "b", "c", "d","d",
                               "g","e","g","d"),
                        to  =c("d", "e", "c", "e", 
                               "g", "f", "f", "g",
                               "h","b","d","a"),
                        weight = c(14,30,25,3,5,6,4,13,2,6,10,10))
g <- graph_from_data_frame(relations, directed=TRUE, vertices=actors)
test.layout <- layout_(g,with_dh(weight.edge.lengths = edge_density(g)/1000))
# 新增edge.curved参数设置曲率
plot(g,vertex.size=30,edge.arrow.size= 0.5,edge.label = relations$weight,
     layout = test.layout, edge.curved = 0.2)

解决方案2：仅双向边设置曲率（更美观） #

如果希望单向边保持直线、仅双向边弯曲，可以通过which_mutual()函数自动识别互反边，针对性设置曲率：

library(igraph)

actors <- data.frame(name=c("a","b","c","d","e","f","g","h"))
relations <- data.frame(from=c("a", "a", "b", "b",
                               "b", "c", "d","d",
                               "g","e","g","d"),
                        to  =c("d", "e", "c", "e", 
                               "g", "f", "f", "g",
                               "h","b","d","a"),
                        weight = c(14,30,25,3,5,6,4,13,2,6,10,10))
g <- graph_from_data_frame(relations, directed=TRUE, vertices=actors)
test.layout <- layout_(g,with_dh(weight.edge.lengths = edge_density(g)/1000))

# 生成曲率向量，默认所有边曲率为0（直线）
edge_curv <- rep(0, ecount(g))
# 给互反边设置0.2的曲率
edge_curv[which_mutual(g)] <- 0.2

plot(g,vertex.size=30,edge.arrow.size= 0.5,edge.label = relations$weight,
     layout = test.layout, edge.curved = edge_curv)

参数说明 #

• 曲率数值可根据显示效果调整，数值越大边的弯曲程度越高，通常0.1~0.3范围内视觉效果最优
• 如果边标签也出现重叠，可以配合edge.label.cex调整标签大小，或edge.label.dist调整标签和边的距离

内容的提问来源于stack exchange，提问作者Mark