Fix stacking bar chart

[yoshiokatsuneo]

What type of PR is this?

• Bug Fix

Description

Fix the issue that stacking bar chart is not stacked.
This issue was introduced by #7514 , and this PR fixes the issue.

How is this tested?

• Manually

I tested by running following query on Query Result datasource. and make stacking bar chart visualization.

select 1 as id, 10 as val, 'a' as grp
union all
select 2 as id, 30 as val, 'a' as grp
union all
select 3 as id, 20 as val, 'a' as grp
union all
select 1 as id, 11 as val, 'b' as grp
union all
select 2 as id, 31 as val, 'b' as grp
union all
select 3 as id, 21 as val, 'b' as grp

Setting:

Before applying this PR:

After applying this PR:

[eradman]

This fix works! Thanks for catching this @yoshiokatsuneo

[yoshiokatsuneo]

@eradman Thank you for your quick review!