I assume you would like to change the spacing between the ticks, which is the same as spacing between the labels.
By default, ChartDirector will spread the labels evenly on the axis. To change the space, you can change the axis length, which by default is the same as the plot area height/width. For you case, you can use a larger or smaller plot area height to change the spacing. (The plot area height is set in the setPlotArea method call in your code.)