Your axis is a time axis. The minimum and maximum values are times, and the major units are time values (hours) as well.
To understand the formatting of the axis, you need to get an understanding of how Excel handles time and date. Internally, time is stored in Excel as a decimal number. A date is a whole number, and a time is stored in decimals of that full number.
The number 1 in date format is Jan-1-1900, 0:00 am. 1,5 is half a day later, i.e. Jan-1-1900 12 noon. So, 0.5 is half a day, or 12 hours. Hence one hour is 0.041666667, 0.25 is 6 hours, etc. A little overview with regards to the values set in your chart axis:
The minimum value on the axis is 6 am, or 0.25. The maximum value is midnight, or 0:00 of the next day, or 1 in General number format. The major unit of the axis is one hour or 0.041666667 in general format.
does that make it clearer?
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