The XY Chart component is based on AM Charts. You can find their documentation in Formatting dates and Formatting date and time. I don't think there's anything in there to help you. You might have to convert to decimal hours and just display as a number.
One last chance: Have a look at Formatting strings. I offered this a solution to forcing numbers to be used as category strings in my answer to Populate Pie Chart from table data - #3 by Alejandro_Alaco.