hi Shwu Hua Gan,
what you need to do is to use 2 charts in conditionally suppressed sections. this is because the axis labels can not be formatted dynamically.
so create a new section below the section where your chart is now, copy your existing chart and then format the scale in the new chart by right clicking on one of the Y axis values and choosing Data Y Axis Options. use the Scale & Numbers tab options to format the axis for this new chart.
now you need to conditionally suppress the original section and the new section. you may want the original section to show up when the Y Axis maximum is going to be greater than 100 so your suppression formula would be something like
maximum({your value field}, {your customer group field}) <= 100
and the suppression formula for the second section would be something like
maximum({your value field}, {your customer group field}) > 100