It's not possible in qv to format an object like in excel. Therefore you need a workaround to come so nearby as possible. One possibility ist to calculate the percentage as an additional field within the script and used it as a normal dimension within the chart. Another possibility could be to calculate these percentage as calculated dimension within the chart.
Otherwise you could use a common "dirty" workaround by using several object beside or overlapping each other - in this case maybe with a textbox which contained the percentage.