If you are creating an XYChart, ChartDirector will not automatically leave a gap in the y-axis. Is it possible you add the gap yourself with Axis.setMargin? If this is the case, please remove the margin.
If you are using a FinanceChart object, it will reserve a gap by default for the legend box. The gap is there even if you do not have a legend box. To disable the gap, please call Axis.setMargin for the y-axis to set the margin to 0.
If you need further help, is it possible to provide the charting part of your code, so I can know how your chart is configured?