Machine Learning in Elixir: berry farm profit graph does not match

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1

In chapter 3 of Machine Learning in Elixir I’m putting in the “BerryFarm” module and plotting the profit graph but my output from VegaLite does not match the plot in the book even though as far as I can tell my code matches.

defmodule BerryFarm do
  import Nx.Defn

  defn profits(trees) do
    trees
    |> Nx.pow(4)
    |> Nx.negate()
    |> Nx.add(Nx.multiply(2, Nx.pow(trees, 3)))
    |> Nx.add(Nx.pow(trees, 2))
  end
end

trees = Nx.linspace(0, 4, n: 100)
profits = BerryFarm.profits(trees)

alias VegaLite, as: Vl

Vl.new(title: "Berry Profits", width: 800, height: 800)
|> Vl.data_from_values(%{
  trees: Nx.to_flat_list(trees),
  profits: Nx.to_flat_list(profits)
})
|> Vl.mark(:line, interpolate: :basis)
|> Vl.encode_field(:x, "trees", type: :quantitative)
|> Vl.encode_field(:y, "profits", type: :quantitative)

I get a graph with a max of 4.409083366394043 and not the ~20 as shown. And for a different number of trees. Looking at the Nx.linspace/3 docs it sure seems like the results should be consistent.