It sounds like the Y-axis (grand total hours worked) should be continuous, not nominal.
So, this is what I think you have, correct me if wrong. Your data has two (relevant) columns: machine ID (nominal) and total hours worked (continuous).
If that is the case, I think Graph Builder would be a better choice. I have attached some mock data (with an attached script) to show how to get your desired graph.
Here are the steps to make the graph.
Go to Graph > Graph Builder
Drag Total Hours to the Y zone.
Click the Line element to get a horizontal line drawn at the mean of the hours worked.
Right-click in the graph and choose Add > Bar.
You should now have a line drawn at the mean and a bar that goes up to that horizontal line.
Drag Machine ID to the X-axis.
This will give you the bar chart, but the line is now connecting the tops of all of the bars.
In the Line element area, click on the variables gray triangle and clear the checkbox for the X-axis (Machine ID), as shown in this picture.
Click Done.
It looks like a lot of steps, but really is not that bad. It just takes awhile to type it all. I made this picture in about 20 seconds.
Dan Obermiller