handle POSIXct frames

plot_ly exhibits strange behavior when trying to loop over a POSIXct variable in an animation. POSIXct variables are handled fine as the axes of a non-animating plot_ly, so I was hoping they would be supported as frames.

It seems the frame is turned into a numeric, which makes sense. However, a POSIXct y axis seems to be mutated beyond use when using a POSIXct frame.

library(plotly)

data <- data.frame(x = 1:100, 
                   t1 = rep(seq.POSIXt(as.POSIXct("2018-04-01"), as.POSIXct("2018-04-10"), length.out = 10), 10),
                   t2 = seq.POSIXt(as.POSIXct("2018-05-01"), as.POSIXct("2018-05-10"), length.out = 100))

data %>%
  plot_ly(x = ~x, y = ~t2, frame = ~t1) %>% 
  add_markers()

Thank you for providing this great capability, and I am excited about the potential for Plotly animations as well!

Start by running the supplied R example with plot_ly, POSIXct t1 frames, and the POSIXct t2 y axis to reproduce the malformed animation. Trace how the frame is converted to numeric and verify that the POSIXct y axis remains usable while animated.