private void checkChart(JFreeChart chart){
  Plot plot=chart.getPlot();
  if (!(plot instanceof PolarPlot)) {
    throw new IllegalArgumentException("plot is not a PolarPlot");
  }
}

private void checkChart ( JFreeChart chart ) { Plot plot = chart . getPlot ( ) ; if ( ! ( plot instanceof PolarPlot ) ) { throw new IllegalArgumentException ( "plot is not a PolarPlot" ) ; } }
test that the chart is using an xy plot with time as the domain axis .
