Hi David, If its of type high charts, you can add advance property plotOptions.series.pointWidth to30. If its a Jfree chart, you need to create a barchart customizer class to control width of bar chart and use the setMaximumBarWidth attribute
CategoryPlot categoryPlot=chart.getCategoryPlot();
BarRenderer renderer=(BarRenderer)categoryPlot.getRenderer();
renderer.setMaximumBarWidth(.30); //Set maximum width of barchart to 30 percent