By definition you cannot (or at least should not) have negative numbers in
this type of chart. It's like trying to show negative numbers in a pie
chart. You could expand the Y axis of your chart so it shows negative
numbers, and the chart will show X% negative and (100-X)% positive. In your
case, 100% doesn't go from 0% to 100%, it goes from -20% to 80%.
Without knowing more about your data, it's anyone's guess how better to show
the data. Perhaps a clustered chart.
- Jon