stat_ecdf does not work

Following code creates thousands of squareds of squares, instead of one step-line.

```
library(plotly)
library(ggplot2)

df <- data.frame(
  x = c(rnorm(100, 0, 3), rnorm(100, 0, 10)),
  g = gl(2, 100)
)

p <- ggplot(df, aes(x)) +
      stat_ecdf(geom = "step")

ggplotly(p)
```