OK, it seems that the y values are TTime. In this case add a TDateTimeIntervalChartSource to the form (the one with the calendar in its icon), link it to the Marks.Source of the Chart's LeftAxis, and set Marks.Style to smsLabel. This way the chartsource will automatically convert the floating-point presentation that you had so far to a time string. The format depends on the date range. In many cases I find the string to be "nicer" when the "SuppressPrevUnit" property is turned off -- otherwise for a series of hour:min label such as 00:01, 00:05, 00:09, 00:20 the hour part ('00') will be displayed only for the first label and suppressed for the following labels as long as the hours do not change (i.e. 00:001, 05, 09, 20).
Look at attached demo which plots some dummy data having a TTime as y.