A pie chart can show all negative values, but only if all the values are negative. It cannot display a mix of positive and negative. A pie chart pretty meaningless when you have a mix of positive and negative values.
If you must use a pie chart, you can modify your expressions like this:
=RangeMax(your_expression, 0) to clip out negatives and display only positives
=RangeMin(your_expression, 0) to clip out positives and display only negatives
Hope that helps