The collection of QualitativeConstantLine objects.
public class QualitativeConstantLineCollection : ChartCollection<QualitativeConstantLine>
The QualitativeAxisX.ConstantLines property returns an instance of QualitativeConstantLineCollection.
A qualitative constant line is a vertical straight line that passes through the chart and indicates an x-axis's value.
Each axis can have any number of constant lines. A qualitative axis (QualitativeAxisX) holds its constant lines in the QualitativeConstantLineCollection collection which the QualitativeAxisX.ConstantLines property returns.