The current reporting functionality does not allow to report by product attribute. This information is saved under 'Discounts' column in the 'Orders' tab after the '|' character, attributes offsets are separated by the '#' symbol as attribute title offset then ':' and value offset.
Therefore, to analyze this data, you will need to use the 'Text to columns' function in Excel and match the offsets to the attribute values in order to create a custom report by attribute.
Having this as pert of the reporting functionality will require development which we will consider for a future version release.
