r中热图的GGplot桶着色

我有一些数据想转换成热图，就像这篇文章中的MSFT一样：

Date temp week wday year
2013-01-02  -21  1   3   2013
2013-01-03  -22  1   4   2013
2013-01-04  -23  1   5   2013
2013-01-07  0  2   1   2013
2013-01-08  25  2   2   2013
2013-01-09  26  2   3   2013
2013-01-10  27  2   4   2013
2013-01-11  28  2   5   2013
2013-01-14  29  3   1   2013
hmap=25

即通过4个十六进制代码立即而不是逐渐地进行更改
使用
cut
并进行分类设置。谢谢-我该怎么做？
Date    temp  week    wday    year
2013-01-02  -21  1   3   2013
2013-01-03  -22  1   4   2013
2013-01-04  -23  1   5   2013
2013-01-07  0  2   1   2013
2013-01-08  25  2   2   2013
2013-01-09  26  2   3   2013
2013-01-10  27  2   4   2013
2013-01-11  28  2   5   2013
2013-01-14  29  3   1   2013

hmap <- read.csv("heatmaptest.txt", as.is=TRUE)
hmap <- transform(hmap,
  week = as.POSIXlt(Date)$yday %/% 7 + 1,
  wday = as.POSIXlt(Date)$wday,
  year = as.POSIXlt(Date)$year + 1900)

library(ggplot2)
ggplot(hmap, aes(week, wday, fill = temp)) + 
  geom_tile(colour = "white") + 
  scale_fill_gradientn(colours = c("#D61818","#FFAE63","#FFFFBD","#B5E384")) + 
  facet_wrap(~ year, ncol = 1)