df <-
  babynames %>% 
  filter(name == "Leslie")

accumulate_by <- function(dat, var) {
  var <- lazyeval::f_eval(var, dat)
  lvls <- plotly:::getLevels(var)
  dats <- 
    seq_along(lvls) %>% 
    purrr::map_dfr(function(x) {
    cbind(dat[var %in% lvls[seq(1, x)], ], frame = lvls[[x]])
      })
}

df <- df %>%
  accumulate_by(~ year)

plot_ly(
    data = df %>% filter(sex == "F"),
    type = "scatter",
    mode = "lines",
    x = ~ year,
    y = ~ prop,
    text = ~ sex,
    frame = ~frame,
    name = "female"
) %>% 
  add_trace(
    data = df %>% filter(sex == "M"),
    name = "male"
  ) %>% 
  animation_opts(
    frame = 20,
    transition = 0,
    redraw = FALSE
  ) %>%
  animation_slider(
    currentvalue = list(
      prefix = "Year "
    )
  ) %>% 
  layout(
    xaxis = list(title = NULL),
    yaxis = list(title = "proportion of sex with name")
  ) %>% 
  config(
    displayModeBar = FALSE
  )