You need to recognize that a column chart uses a category X axis, not a value axis. So, when the XY scatter is laid on top of the column chart, the categories for the column chart will correspond to the X values 1, 2, 3, etc. You will need to figure out what value to put in A15 and A16 so that the line is between category 3 and category 4. You can probably work out an interpolative formula, but I expect the value for A15 and A16 will be close to 3.8.