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Grouped bar chart issue

I try to plot a grouped bar chart, by using python and the javascript lib plotly. My code looks like this:

import plotly.graph_objs as go
import plotly

traces = []

y = [[0, 2, 4, 6], [1, 3, 5, 7]]

for single_y in y:
    traces.append(go.Bar(
        x=['giraffes', 'orangutans'],
        y=single_y
    ))

layout = go.Layout(
    barmode='group'
)

fig = go.Figure(data=traces, layout=layout)

plotly.offline.plot(fig)

However, the result is not as expected. Because the y list of lists contains 4 values and therefore I would expected that all 4 values are mapped to the two x columns automatically.

What am I doing wrong here?