通过在因子中重新排列级别，可以更改图例标签的顺序.运行:

By reordering the levels in your factor, you can change the order of the legend labels. Run:

df$var2 <- factor(df$var2, levels=c("levelB", "levelC", "levelA"))

然后重新运行ggplot代码，现在levelB应该位于图例的顶部，绿色，levelC第二和红色，levelA第三和黑色.

Then rerun the ggplot code, and levelB should now be at the top of the legend and green, levelC second and red, and levelA third and black.