in my practice, I usually present Waterfall charts using positive amounts and manage their direction (up or down) by calculating the offset accordingly. This way, all bars carry positive numbers, and the numbers are always presented on top of the bars.
You can also try experimenting with Dual() values - your numeric value can be positive, while text representation may look like negative.
Finally, you can experiment with an extra set of Expressions - those with no visual Representation, except for "Values on Data Points".
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You probably need to check if your value is negative, then multiply by -1 to get a positive number.
Positive numbers are shown on the top, negative on the bottom of a bar chart.
You may need to adjust the bar offset to take care of the changed sign as well as creating a dual value with a text representation that shows your negative original value as text.