ggplot2:删除颜色并在条形图中绘制边框

我目前正在使用ggplot2绘制一个条形图,其中填充的级别太多.结果,我无法判断一个栏结束而另一个开始.

这是我现在正在做的样本数据代码.

variable<-c("X1","X1","X1","X1","X1","X1","X1","X1","X1","X2","X2","X2","X2","X2","X2","X2","X2","X2","X3","X3","X3","X3","X3","X3","X3","X3","X3")
Length.1<-c(4.24,0.81,0.81,NA,NA,NA,NA,NA,NA,4.24,0.81,0.81,NA,NA,NA,NA,NA,NA,4.24,0.72,0.72,0.16,NA,NA,NA,NA,NA)
data<-data.frame(variable=as.factor(variable),value=as.factor(1:length(variable)),Length.1=Length.1)

## Plots a stacked bar chat in ggplot2 with text labels. Credit to MYaseen208
library(ggplot2)
p <- qplot(variable, Length.1, data = data, geom = "bar", fill = value,     theme_set(theme_bw()))
p + geom_text(aes(label = Length.1), size = 3, hjust = 0.5, vjust = 3, position =     "stack") 

目前,情节看起来很好.然而,在我的实际数据集中,length(data$value)要高得多,我无法清楚地看到不同条形图之间的差异.因此,我希望将条形图的颜色更改为白色,并在每个条形图周围绘制黑色边框.有谁知道我怎么做到这一点？