frame in `ggplotly` not working when taking a `Date` object

Hi Carson,

The new animation feature is awesome. But I run into an issue when the frame arg takes a Date object, and I need to convert to numerics. Please see the little reproducible example below:

library(plotly)
df <- data.frame(
  Date = seq(as.Date("2017-01-01"), as.Date("2017-01-05"), by = 1),
  Hour = rep(1:24, 5),
  Value = rnorm(24 * 5)
)
p1 <- ggplot(aes(Hour, Value, frame = Date), data = df) +
  geom_line() # not working
ggplotly(p1)
p2 <- ggplot(aes(Hour, Value, frame = as.numeric(Date)), data = df) +
  geom_line() # working
ggplotly(p2)

Thanks a lot,
Earo

运行 issue 中可复现的 R 示例，并比较 frame 使用 Date 的 ggplotly(p1) 与使用 as.numeric(Date) 的 ggplotly(p2)。从 ggplotly 对 frame aesthetic 的处理入手，追踪 Date 值的转换过程。当使用 Date 值的 frame 无需手动进行数值转换即可正常工作，且现有的数值情况仍能正常工作时，即可视为完成。