在ggplot2中为每个面板添加一条具有不同截距的垂直线

I'm using ggplot2 to create panels of histograms, and I'd like to be able to add a vertical line at the mean of each group. But geom_vline() uses the same intercept for each panel (i.e. the global mean):I'm using ggplot2 to create panels of histogram