code: public Histogram getResidualPlot(int labelClassIdx) {
          String title = "Residual Plot - Predictions for Label Class " + labelClassIdx;
          int[] counts = residualPlotByLabelClass.getColumn(labelClassIdx).dup().data().asInt();
          return new Histogram(title, 0.0, 1.0, counts);
      }
docs: Get the residual plot, only for examples of the specified class.. The residual plot is defined as a histogram of<br>
  |label_i - prob(class_i | input)| for all and examples; for this particular method, only predictions where
  i == labelClassIdx are included.<br>
  In general, small residuals indicate a superior classifier to large residuals.
  
  @param labelClassIdx Index of the class to get the residual plot for
  @return Residual plot (histogram) - all predictions/classes