可以用ggupr包中的multiplot函数
multiplot(…，plotlist=NULL, cols=1,layout=NULL)
plotlist: a list of ggplot objescts
cols: Number of columns in layout
layout: A matrix specifying the layout. If present, 'cols' is ignored
If the layout is something like matrix(c(1,2,3,3), nrow=2, byrow=TRUE), then plot 1 will go in the upper left, 2 will go in the upper right, and 3 will go all the way across the bottom
注意，一般默认是按列填充，如设定图片类型为两行，则先填充第一列，再填充第二列，以此类推。
例：multiplot(p1,p2,p3,p4,p5,p6,p7,p8,p9,p10,cols=5)
