只需添加将y映射到其他变量的geom。为简单起见，我将一些美学元素移至geom_boxplot。

Simply add an geom mapping the y to a different variable. For the sake of simplicity, I moved some of the aesthetics to the geom_boxplot.

MyPlot <- ggplot(MyData, aes(x=Month)) + geom_boxplot(aes(y=Note, fill=Treatment)
MuPlot <- MyPlot + geom_pointline(aes(y=Optimum), colour="green", stroke="black")

但是，这不会为您添加图例，因为ggplot2不支持相同比例的多种编码（即同时使用处理和单独的颜色变量）。

This will however not add you points to the legend, as ggplot2 does not support multiple encodings of the same scale (i.e. using both Treatment and a separate variable for colour).

geom geom_pointline 来自柠檬

The geom geom_pointline is from the 'lemon' package.

第二点，尝试第二行：

MuPlot <- MyPlot + geom_pointline(aes(y=Optimum, colour="Optimum"), stroke="black") + scale_colour_manual(values('Optimum'='green'))