For 126 consecutive days, a process engineer has measured the temperature of champagne bottles as they are made ready for serving. Each day, she took a sample of 8 bottles. The average across all 1,008 bottles (126 days, 8 bottles per day) was 58 degrees Fahrenheit. The standard deviation across all bottles was 1.5 degree.

When constructing an X-bar chart, what would be the center line?