x::AbstractVector
y::AbstractVector
group::AbstractVector

df = readtable(Pkg.dir("Vega", "vega-datasets/streamdata.csv"))
a = streamplot(x = df[:date], y = df[:value], group = df[:key])
colorscheme!(a, palette = ["#B30000", "#E34A33", "#FC8D59", "#FDBB84", "#FDD49E", "#FEF0D9"])