as line chart by default starts on axis, and your column width is more than 0, then some part of first and last column will be outside plot area (incase you don't set StartOnAxis = false Mask will cut off the part outside the plot area. Set to false if you don't want this.

public bool VerticalLinesMasked { get; set; }

Public Property VerticalLinesMasked As Boolean

public:
property bool VerticalLinesMasked {
	bool get ();
	void set (bool value);
}