The Keywords chart found on the overview "view" in Product Research is designed to compare the performance of eBay title keywords.
In order to appear in the chart, keywords must meet a minimum frequency when compared against the following:
The number of unique listing titles found by the search
The number of days in the date range of the search.
If the date range of the search is very long, or the listing titles found by the search have few keywords in common, the report may find no keywords with significant frequency and the Keywords chart may not appear.