Adding a label without another column

Hi Everyone,
I have an excel with prices of 2 years, and columns charts, where for each product 2 columns for each year.
I want to add a label with the difference at prices between the 2 years. Not another column, only another label.
How do I do that?

You can see at the attachment, for each fruit written the price at each year.
How to add to the column chart a lebel of the price difference, e.g. for the grapes to add a label of 1 $/Kg, wich is the difference at prices between years. And do it without adding another column, only a label.

Thanks,
Koby