Here is the solution to your problem. You can view the chart along with code here. I am posting it here as well
# don't switch to scientific notation, since we want date to be
# represented in milliseconds
options(scipen = 13)
dat = transform(df, Date2 = as.numeric(as.POSIXct(Date))*1000)
library(rCharts)
h1 <- hPlot(Numbers ~ Date2, data = dat,
group = 'Type',
type = "line",
radius=6
)
h1$xAxis(type = 'datetime', labels = list(
format = '{value:%Y-%m-%d}'
))
h1