public static DataFrame<Object> randomData(final double utilization) {
   final DataFrame<Object> df = new DataFrame<Object>("name", "value", "category");
   for (int i = 0; i < MILLIONS || memoryUtilization() < utilization; i++) {
     df.append(randomRow());
   }
   return df;
 }
 public static DataFrame<Object> randomData(final int rows) {
   final DataFrame<Object> df = new DataFrame<Object>("name", "value", "category");
   for (int i = 0; i < rows; i++) {
     df.append(randomRow());
   }
   return df;
 }