如何在scale_x_date中定义date_breaks的原点

Question

请考虑以下数据并绘制。每三个月定义一次date_breaks，第一个休息时间是feb。但我想从jan开始。它每两个月工作一次，请参阅第二个图。我试图指定限制（请参见代码）无济于事。

library(ggplot2)

df_foo <- data.frame(date = as.Date(c('2008-06-01','2008-07-01','2008-07-01','2009-04-01','2009-04-01','2010-04-01')))

ggplot(df_foo, aes(x = date)) + 
  stat_count(geom = 'line', aes(y = ..count..)) +
  scale_x_date(date_breaks = '3 months', date_labels = '%b', 
               limits = c(as.Date('2008-01-01'), as.Date('2010-12-01'))) 

ggplot(df_foo, aes(x = date)) + 
  stat_count(geom = 'line', aes(y = ..count..)) +
  scale_x_date(date_breaks = '2 months', date_labels = '%b', 
               limits = c(as.Date('2008-01-01'), as.Date('2010-12-01'))) 

^{由reprex package（v0.2.1）于2018-11-29创建}