How can I add rows to grouping in views?
The views Format Settings has your answer.  There is a drop-down for "Grouping field Nr.1" that does exactly this.



As it is, you have one "row" showing for each unique set of values because this is what mysql returns.  By setting this field grouping, Views will match up the values from the selected field and use them as headers for sub sections of your results.

If this is not a good enough match to your issue, please provide details on how it doesn't work and I can update the answer.



Continued:

The view is a tool for making a mysql query and then theming the returned result.  With this in mind, you need to craft a set of data relations and filters to get the appropriate data linkages for the ratios you want in your display.  This means you want the database handing you a field table like:

Category parent | product | category | trait


Sweet Corn | corn product 1  | Type 1  |  Null
Sweet Corn | corn product 2  | Type 2  |  Null
Sweet Corn | generic corn product  |  Null | Augmented
Onions | onion product 1 | Type 3 | Null
Onions | onion product 3 | Null  | Celebrate


Then once you have the dataset you need, you work on the theming side of the views tool.  Use the above mentioned Grouping field setting on the Category Parent.  This will make the sections with the parent titles as indicated in your question, but it might look like 


Sweet Corn 


corn product 1  | Type 1  |  
generic corn product  | | Augmented  



Then use the "exclude from display" on the product field, or actually remove it from the fields list.  I have it included here because it is needed to show the data grouping as I understand it in your question.  Finally, configure the "category" field with the link you want it to have and exclude it from display; use the "No Results Behavior" rewrite option on Trait to put in the token for the Category field, something like [field_term_category].  You should be able to find this token in the list of "replacement patterns" available in the Rewrite Results area.

 

You should now have a list of parent categories showing the linked sub categories and traits where you want them.  The "trait" field will either have the category or the trait in there, as it shows trait when trait is present, and category when not.  Be sure to uncheck the "hide if empty".