r - 将热图映射到Calendar Heatmap上的网格线

Question

我按照此website中显示的日历热图的示例（您必须向下滚动到日历热图部分）

使用网站上的示例：

library(ggplot2)
library(plyr)
library(scales)
library(zoo)

df <- read.csv("https://raw.githubusercontent.com/selva86/datasets/master/yahoo.csv")
df$date <- as.Date(df$date)  # format date
df <- df[df$year >= 2012, ]  # filter reqd years

# Create Month Week
df$yearmonth <- as.yearmon(df$date)
df$yearmonthf <- factor(df$yearmonth)
df <- ddply(df,.(yearmonthf), transform, monthweek=1+week-min(week))  # compute week number of month
df <- df[, c("year", "yearmonthf", "monthf", "week", "monthweek", "weekdayf", "VIX.Close")]

# Plot
ggplot(df, aes(monthweek, weekdayf, fill = VIX.Close)) + 
  geom_tile(colour = "white") + 
  facet_grid(year~monthf) + 
  scale_fill_gradient(low="red", high="green") +
  labs(x="Week of Month",
   y="",
   title = "Time-Series Calendar Heatmap", 
   subtitle="Yahoo Closing Price", 
   fill="Close")

这是您获得的日历热图。

我可能会在这里特别挑剔，但有没有一种方法可以让条形符合刻面的网格线，同时将标签保持在中点？

我已经尝试过使用axis（），按照this这样的帖子提出建议，但是当我尝试将其翻译为日历热图时，我觉得有些东西会丢失。

任何建议都很棒，谢谢！