How do you do this in Excel? I am looking for the option but not finding it. I am not a power user of Excel, though.
I don't know why you are adverse to creating new columns. The typical way to do it for a single series in Numbers has been suggested already, use a formula to create a column of the positive values, another formula to create a column of the negative values, and plot both columns. I prefer a stacked bar chart because it preserves the correct bar spacing on the chart.
But you are plotting two columns of data already. For that, there is no good solution. You would have to create two stacked bar charts and overlay them to look like one. We showed that recently in another thread.