You have to make the category axis based on values 1, 2, 3.
When you change the chart type for Target Days to XY Scatter, switch the X-values and Y-values.
Then make the secondary vertical axis run from 0.5 to 3.5 to match the layout of the primary vertical axis.
Finally, delete the secondary vertical axis.
See the sample workbook at https://www.dropbox.com/s/v8oojvaxo7k02fj/CombiChart.xlsx?dl=1