private void recalcMinMaxFromTo() {
		min = null;
		max = null;
		from = null;
		to = null;

		Enumeration<Double> ks = values.keys();
		while (ks.hasMoreElements()) { 
			Double k = ks.nextElement();
			Double v = values.get(k);
			if (min == null || v < min) min = v;
			if (max == null || v > max) max = v;
			if (from == null || k < from) from = k;
			if (to == null || k > to) to = k;
		}
	}