Using Excel's built in sexagessimal notation, it should be fairly easy to switch from "time of day" to "elapsed hours" -- elapsed time number formats use brackets -- a time format like [hh]:mm:ss should show elapsed hours.

If you want your chart to show elapsed decimal hours rather than using the sexagessimal notation, you will need to convert your time data (fraction of a day) in the spreadsheet to elapsed hours by multiplying everything by 24 (to convert days to hours) then formatting as a suitable number format.