how to show group parent as one cell (by combining first columns of children rows)

I would love to manage to see the parent cell as one united cell as i showed on the screenshot below. I couldn't find solution from themeing, customization docs. Could you please tell me what is the proper way of achieving this? Thanks

Here you are

https://plnkr.co/edit/oOtlvoTPuaIDB2Kd?preview

This is the closest solution I can achieve

Basically, we use rowSpan feature. In this example, we have 2 groups; country and year respectively. To get the desired result, we add a rowSpan method like that

rowSpan: (params) => {
  // when all group columns collapsed
  if (params.node.field === 'country') {
    return 1;
  // when first level is expanded
  } else if (params.node.field === 'year') {
    return (
      params.node.parent.childrenAfterGroup.length -
      params.node.childIndex
    );
  // when second level is expanded
  } else {
    // if it is the latest group, our calculation logic changes
    if (params.node.parent.lastChild) {
      return (
        params.node.parent.childrenAfterGroup.length -
        params.node.childIndex
      );
    } else
      return (
        params.node.parent.childrenAfterGroup.length -
        1 +
        params.node.parent.parent.childrenAfterGroup.length -
        params.node.childIndex -
        params.node.parent.childIndex
      );
    }
  }
},

The idea is to count rows regarding the row child index. You need to observe the code and understand it, it's somewhat complicated.

Result

UPDATE

If you want to have a background color only if the country column is expanded, we should add another condition to check it this way

if (params.node.parent.expanded)

Here's the updated plunker: https://plnkr.co/edit/oOtlvoTPuaIDB2Kd?preview