it "combines data sources" do
      ds = Ahoy::DataSource.new
      ds.add "foo", { january_first => 2, january_second => 1 }
      ds.add "bar", { january_first => 1, january_third => 2 }
      expect(ds.build).to eq :x => ["2015-01-01", "2015-01-02", "2015-01-03"],