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Heatmaps failed to produce the expected outpuy

I am unable to reproduce the plots for the examples given in here it displays nothing on the plot
For example for this code from the same link:

import plotly.graph_objects as go

fig = go.Figure(data=go.Heatmap(
                   z=[[1, None, 30, 50, 1], [20, 1, 60, 80, 30], [30, 60, 1, -10, 20]],
                   x=['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'],
                   y=['Morning', 'Afternoon', 'Evening'],
                   hoverongaps = False))

It is same for the px.imshow

What else should I add to the given example so that i can plot the correlation matrix?