layer_bars does not work if `x` and `y` are not initially specified in `ggvis` call

[ttzhou]

layer_bars does not work if `x` and `y` are not initially specified in `ggvis` call

[ttzhou]

In layer_bars:

function (vis, ..., stack = TRUE, width = NULL) 
{
    new_props <- merge_props(cur_props(vis), props(...))
    check_unsupported_props(new_props, c("x", "y", "x2", "y2"), 
        c("enter", "exit", "hover"), "layer_bars")
    x_var <- find_prop_var(new_props, "x.update")
    discrete_x <- prop_countable(cur_data(vis), new_props$x.update)
    vis <- set_scale_label(vis, "x", prop_label(**cur_props**(vis)$x.update))
    ...
}

Would setting cur_props(vis) to new_props(vis) rectify this?