How do I force showing '+' sign using StringFormat

Question

Is there a way to force showing the + sign in front of positive numbers when using StringFormat?

For example:

<TextBlock Text="{Binding Path=PercentAgainstBudget, 
                          StringFormat={}{0:0.00}%}" />

If PercentAgainstBudget is negative I see the - sign. But if its positive, it does not. Since this number is an offset, I'd like to force showing +/-. I could make a ValueConverter but I'm wondering if there's a way to do it through the StringFormat property.