1.2.1节有如下程序，
#1.2.1 Undirected Graphs
library(gRbase)
#下面四句是等价的
ug0 <- ug(~a:b, ~b:c:d, ~e)
ug0 <- ug(~a:b+b:c:d+e)
ug0 <- ug(~a*b+b*c*d+e)
ug0 <- ug(c("a","b"),c("b","c","d"),"e")
ug0

紧接着有
library(Rgraphviz)
plot(ug0)

为什么我的运行了plot(ug0)画不出来图，并有以下错误
Error: Layout type: "dot" not recognized. Use one of:
Warning messages:
1: In plot.xy(xy.coords(x, y), type = type, ...) :  "len" is not a graphical parameter
2: In plot.xy(xy.coords(x, y), type = type, ...) :  "edgemode" is not a graphical parameter
3: In plot.xy(xy.coords(x, y), type = type, ...) :  "len" is not a graphical parameter
4: In plot.xy(xy.coords(x, y), type = type, ...) :  "edgemode" is not a graphical parameter
5: In plot.xy(xy.coords(x, y), type = type, ...) :  "len" is not a graphical parameter
6: In plot.xy(xy.coords(x, y), type = type, ...) :  "edgemode" is not a graphical parameter
7: In plot.xy(xy.coords(x, y), type = type, ...) :  "len" is not a graphical parameter
8: In plot.xy(xy.coords(x, y), type = type, ...) :  "edgemode" is not a graphical parameter
There were 16 warnings (use warnings() to see them)



所有的运行是
> library(gRbase)
> ug0 <- ug(~a:b, ~b:c:d, ~e)
> ug0 <- ug(~a:b+b:c:d+e)
> ug0 <- ug(~a*b+b*c*d+e)
> ug0 <- ug(c("a","b"),c("b","c","d"),"e")
> ug0
A graphNEL graph with undirected edges
Number of Nodes = 5
Number of Edges = 4
> library(Rgraphviz)
> plot(ug0)
Layout type: "dot" not recognized. Use one of:
Warning messages:
1: In plot.xy(xy.coords(x, y), type = type, ...) : "len" is not a graphical parameter
2: In plot.xy(xy.coords(x, y), type = type, ...) : "edgemode" is not a graphical parameter
3: In plot.xy(xy.coords(x, y), type = type, ...) : "len" is not a graphical parameter
4: In plot.xy(xy.coords(x, y), type = type, ...) : "edgemode" is not a graphical parameter
5: In plot.xy(xy.coords(x, y), type = type, ...) : "len" is not a graphical parameter
6: In plot.xy(xy.coords(x, y), type = type, ...) : "edgemode" is not a graphical parameter
7: In plot.xy(xy.coords(x, y), type = type, ...) : "len" is not a graphical parameter
8: In plot.xy(xy.coords(x, y), type = type, ...) : "edgemode" is not a graphical parameter
There were 16 warnings (use warnings() to see them)


谢谢！