Customer wants to add horizontal line to a combination chart with dual scale is on.
So if he add a line with value of 6, then there will be 2 line drawn in the chart for each of the 2 scales most likely in different ranges, e.g. one with range of 0 to 10, the other with range of 0 to 15.
Customer only wants to show 1 horizontal line for the scale either on the left or on the right.