To set the bar width on Bar Chart

Hi, I have created a bar chart report, I want to set the width of the bar in inches dynamically based on the number of bars. So I have set the bar width in beforeGeneration method using chart.setUnitSpacing(), but the bar width is not exactly matching, have following questions: 1. In which unit do I need to set the unit spacing (either in points/pixels/inches)?
2. How to set the width of the bar in inches?