R语言ggplot画双坐标-柱状图、折线图

ggplot一般不用来画双坐标，有些图双坐标结合起来看比较好，比方说条形图和柱状图。
1.画双坐标要用到函数scale_y_continuous，这个函数规定次坐标与主坐标的换算关系，用 sec_axis(~./57.69231）来设置，其中57.69231这个换算系数可以根据实际情况调整；
2.画了双坐标，需要指定谁在次坐标上显示，那么在次坐标要缩小的倍数，在需要显示在次坐标的数据就需要提升多少倍，这样的话，才能正常显示。

dummy <- data.frame(item = c("X","X","X","X","Y","Y","Y","Y"),year = c(2017,2018,2019,2020,2017,2018,2019,2020),
                    X1 = c(500,900,600,700,700,1200,900,1500),
                    X2 = c(15,18,23,11,26,23,13,15)
)

ggplot(dummy, aes(x = year, group = item, fill = item, color = item)) +
    geom_bar(aes(y = X1), stat = "identity", position = 'dodge2') +
    #geom_line(aes(y = X2*57.69231)) +
    geom_line(aes(y = X2*57.69231)) +
    scale_y_continuous(sec.axis = sec_axis(~./57.69231, name = "percent"))

执行结果：

如果在geom_line这行代码去除*57.69231，执行图片异常：

 geom_line(aes(y = X2)) +