The answer actually is very easy. Tough most of the charts can figure out the required size on their own, the sankey chart obviously can’t. So basically you just have to set dimensions for all exports on sankey charts (yes even for vector graphics like eps and svg).
Also worth mentioning is that a minimum size is required. While my example now looks satisfying with 1920×1080, a size of 1280×720 looks broken even with vector-graphics.
fig = go.Figure(...) fig.update_layout(width=1920, height=1080) fig.write_image(...)
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