引自
https://stackoverflow.com/questions/1330989/rotating-and-spacing-axis-labels-in-ggplot2/1331400#1331400?newreg=534575cfc30344c797e7f207b0e6b8ac
使用theme函数来解决这个问题
q + theme(axis.text.x = element_text(angle = 90, hjust = 1))
原文：http://www.cnblogs.com/yumtaoist/p/4372312.html