I d like to create a sankey chart chart using vega in Deneb. I found an article online and I have the following code, but I would like to show the countries grouped

currently my result is like this, I could not add the gap correctly on both sides;

here is my code..

https://vega.github.io/editor/#/gist/a6c6f4b41a20fd763d08ed519ba1fcda/Vega_Sankey_Chart_with_issue.json

can you please help me .
Thank you.