So you only want to change the '-1' in '1'? If so you can do the following:
-Go to the properties of your chart.
-Click on the 'notations' tab
-Click on your expression
-There is a field with something like this '#.##0,00- '
-Just remove the '-' to make it like this '#.##0,00'
If this solves your problem then the credits must go to the user gwassenaar who gave this solution in another topic.