如何将基于日期的颜色渐变应用于ggplot2散点图？

Question

在下面的例子中,我希望最近的点是更深的蓝色阴影(或者更不透明).

data <- data.frame(x=c(1,2,3,4,5,6,7,8,9,10,11,12), 
    y=c(1,2,3,4,5,6,7,8,9,10,11,12), 
    dt=c("2010-12-31","2010-11-30","2010-10-29","2010-09-30",
        "2010-08-31","2010-07-30","2010-06-30","2010-05-31","2010-04-30",
        "2010-03-31","2010-02-26","2010-01-29"))
p <- ggplot(data, aes(x,y,color=dt)) + geom_point() + 
    scale_colour_gradient(limits=c("2010-01-29","2010-12_31"), 
        low="white", high="blue")
print(p)

Answer 1

dt是因子变量,并且可能缩放_*_渐变不可用于离散变量.

你可以将dt转换为Date,然后转换为连续变量的整数.

这是一个例子:

ggplot(data, aes(x,y, colour=as.integer(as.Date(data$dt)))) + 
  geom_point() + 
  scale_colour_gradient(limits=as.integer(as.Date(c("2010-01-29","2010-12-31"))),
                        low="white", high="blue") +
  opts(legend.position="none")