tooltips for line charts to show a tip for every point in a line

[saptarshiguha]

Hello,
I have code like

u=vegalite(title="FOO",viewport_width=320,viewport_height=260) %>%
  add_data(s)  %>%
  encode_x("date", "temporal") %>%
  encode_y("PctOn", "quantitative") %>%
  mark_line()

I'm using vegalite installed via devtools. When I hover over the line, the tool tip (really nice, thanks!) shows only the values from the last point in the series. The data set has 90 values, would it be possible to show the tooltip for every value?

Thanks much