Even though I am still running 2007, I have seen this mentioned on the forum many times. It seems that many/most/all of Excel's specialty charts are not very flexible.

If you cannot find a way to make your combo chart using the built in specialty chart, remember that most if not all of those specialty charts can be made as standard chart types that are more flexible. Waterfall charts, for example, can be made as either line charts (with up/down bars -- probably easier in your case where some of the values are negative) or stacked column charts. Either way, using a basic chart type should allow you to regain the flexibility needed to add a horizontal line. Tutorial for creating waterfall charts: https://peltiertech.com/excel-waterf...bridge-charts/

In this case, I wonder if the built in chart could simply be formatted to have the horizontal axis fixed at 0 rather than be at the bottom of the chart (for some reason, the position of the horizontal axis is considered a property of the vertical axis -- the "horizontal axis crosses at" property). If that were possible, you could fix the position of the horizontal axis at the y=0 value, then format the axis to be thick and red and have the axis labels at the bottom.

A couple of possible approaches. Obviously, the second one would be easier. If necessary, the first one will work, but will require the extra work needed to create the waterfall chart using basic chart types.