I think there are multiple approaches to tackle this issue and it depends a bit on how you want the final plot to look like.

Approach 1: Helper Column

The potentially easier solution is to simply add an extra column to your data with, e.g., the mean value for which you want to have the horizontal line. Assuming your regular data is stored in the range A1:A11 and the addition of the values for the horizontal line in column B (these values can of course link to your computations of the e.g., the mean):

Select the data -> Insert -> Clusterd Column Chart. Next select your Chart -> Chart Design -> Change Chart Type -> Combo -> Change the H1 chart type to Line.

This will return the following chart:

Approach 2: Helper Table

Another approach is using a helper table that stores the information for the horizontal line, e.g., in the range D1:E3:

Select now your values in column A and the x value in column C -> Insert -> Recommended Charts -> Combo -> values to Clustered Column and x to XY Scatter with Smooth Lines. Next, select the chart -> Chart Design -> Select Data -> Select x -> Edit -> select x and y values according to your helper table.

This will return the following chart:

The main benefit of this slightly more complex approach is that you can better tailor the length of the horizontal line.

The names in the legends for the helper columns can be changed accordingly. The same goes for adding additional horizontal lines.