Default mark_area is upside down

[hadley]

df <- data.frame(x = 1:10, y = runif(10))
df %>% ggvis(~x, ~y) %>% layer_ribbons()

[wch]

I think that y2 needs to be listed as a required prop for ribbon/area:

df <- data.frame(x = 1:10, y = runif(10))
df %>% ggvis(~x, ~y, y2=0) %>% layer_ribbons()

[hadley]

It should be required for ribbon, but it shouldn't be needed for area.

[wch]

We'll need to re-think where we do props checking and applying the defaults in general.