I've been looking over this post and the attached workbook to try and help you out, but I'm not clear on the question. You say "the hires from 2015 are not displayed" but they seem to be displaying fine in the screenshot and workbook. The Business Dates for the 2015 hires are April 1 2017 and September 1 2017, and in the chart there's a red -1 for each of those months' columns.
Playing around in the workbook, I get the same results when swapping Business Date for Hire Date--the 2015 hires are present, but now they're both lumped under September 2015, as they should:
Could you update this thread with some more details about the problems you're seeing, and what you're trying to accomplish?