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2D histogram differences of plots?

I’m trying to plot the difference of two plotly 2-D histograms.

chart1 = go.Histogram2d(x=sev_df.dt, y=sev_df.aam,histnorm='enumerated'...
chart2 = go.Histogram2d(x=sig_df.dt, y=sig_df.aam,histnorm='enumerated'...

After the charts are created, I would like to plot the difference between the 2-d histograms. Basically, I need to access the ‘Z’ values used for the plots and find the difference.

I am not able to find any leads or examples. Any ideas?