绘制树状图

Question

我想通过grid.arrange函数合并来自聚类结果的两个树形图。我使用的代码是

d.a <- dist(df.a, method = "euclidean")
# Hierarchical clustering 
res.hc.a <- hclust(d.a, method = "ward.D2" )

d.b <- dist(df.b, method = "euclidean")
# Hierarchical clustering 
res.hc.b <- hclust(d.b, method = "ward.D2" )

p1=plot(res.hc.a, cex = 0.75,main="a")
rect.hclust(res.hc.a, k = 6, border = 2:7)

p2=plot(res.hc.b, cex = 0.75,main="b")
rect.hclust(res.hc.b, k = 7, border = 2:8)

grid.arrange(p1, p2, ncol = 2, top  = "Dendrograms")

但是我遇到了像这样的错误

Error in gList(list(wrapvp = list(x = 0.5, y = 0.5, width = 1, height = 1,  : 
  only 'grobs' allowed in "gList" 

问题是grid.arrange函数不考虑每个绘图代码的第二行。我不知道如何解决这个问题。我的第二个问题是我想要显示每个树形图而不是水平线。有什么想法吗？