The concept of section access itself, to restrict a data.
you have applied a section access to plant level and you want show the data of other plants? then why you have applied a section access?
For handling the your situation, you have to create a one more table which contains other plants sales and map to the section access table.
IF YOU APPLIED A section access to Plant, then user A will see only Plant A data only which user a has applied in the section access table.
Now create one more table in the data modelling like,
Plant otherPlant Sales
A B 2
A C 3
A A 1
and pass otherplant in your chart.
To See Plant level values for all plant you can create Group wherein each plant will belong to all other plant available in your data through this each plant will be able to see other plants data while loading.
P.S This will remove the data reduction as mentioned earlier as each plant will get access to other plant data.
Hope this solve the issue.