Nyaplot uses index as default axis for vector but not for dataframe

[baarkerlounger]

df = Daru::DataFrame.new({y1: [11, 15, 8], y2: [9, 10, 3]}, 
     index: ["a", "b", "c"])

df.plot type: :bar, y: :y1 do |plot, diagram|
  plot.y_label "Frequency"
  plot.x_label "Category"
  plot.yrange [0,20]
end

Throws NoMethodError: undefined method to_sym' for nil:NilClass`

I would expect it to behave like Daru::Vector and take the index as default axis.

dv = df[:y1]

dv.plot type: :bar, y: :y1 do |plot, diagram|
  plot.y_label "Frequency"
  plot.x_label "Category"
  plot.yrange [0,20]
end