@Override
		public void add(final double value) {
			++frequencies[partition.indexOf(value) + 1];
			observer.accept(value);
		}
Set an additional value observer.

@param observer the value observer
@throws NullPointerException if the given {@code observer} is
        {@code null}.
@return {@code this} builder
jenetics.incubator/src/main/java/io/jenetics/incubator/stat/Histogram.java