You create a constant line by creating an instance of the NAxisConstLine class. The following code will add a constant line to the PrimaryY axis of the first chart in the control:

NCartesianChart chart = (NCartesianChart)NChartControl1.Charts[0];
NAxis axis = (NAxis)chart.Axis(StandardAxis.PrimaryY);
NAxisConstLine constLine = new NAxisConstLine();
axis.ConstLines.Add(constLine);

Dim chart As NCartesianChart = CType(NChartControl1.Charts(0), NCartesianChart)
Dim axis As NAxis = chart.Axis(StandardAxis.PrimaryY)
Dim constLine As New NAxisConstLine
axis.ConstLines.Add(constLine)

After you create the constant line you can modify its Value property which associates the constant line with a specific axis value. The following code will display the newly created line on value 20 of the PrimaryY axis:

constLine.Value = 20;

constLine.Value = 20

You can also instruct the const line to include the value in the range of the axis the const line is attached to. This is done with the IncludeInAxisRange property:

Constant lines can be displayed as simple lines or as planes. The style of the constant line is controlled through the Style property of the NAxisConstLine class. It accepts the values of the ConstLineStyle enumeration and by default is set to Line. The following code switches the constant line style to Plane:

constLine.Style = ConstLineStyle.Plane;

constLine.Style = ConstLineStyle.Plane

When the constant line is visualized as line you can control the appearance of the line with the help of a NStrokeStyle object accessible trough the StrokeStyle property. The following snippet shows how to change the line color to blue:

constLine.Style = ConstLineStyle.Line;
constLine.StrokeStyle.Color = Color.Blue;

constLine.Style = ConstLineStyle.Line
constLine.StrokeStyle.Color = Color.Blue

When the constant line is visualized as plane you can also control the filling of the plane with the help of a NFillStyle object accessible trough the FillStyle property. In this case the StrokeStyle property controls the border of the plane. The following code will display a constant plane with red filling and black border:

constLine.Style = ConstLineStyle.Plane;
constLine.FillStyle.SetSolidColor(Color.Red);
constLine.StrokeStyle.Color = Color.Black;

constLine.Style = ConstLineStyle.Plane
constLine.FillStyle = New NColorFillStyle(Color.Red)
constLine.StrokeStyle.Color = Color.Black

By default constant lines are displayed across the entire chart area in their respective dimension. Sometimes you may need to specify a begin and end value of the constant line on a reference axis (parallel to the constant line). To do this you have to set the EnableBeginEnd property to true. Afterwards you need to set the desired begin/end value through the BeginValue and EndValue properties. Depending on which axis the constant line belongs to you can specify a reference axis at which the begin and end values will be scaled. This is achieved with the help of the BeginEndAxis property. Constant lines contained in vertical axes accept as a parameter only horizontal axes. Constant lines contained in a horizontal or depth axis accept the vertical axes as reference axes.

The following code will display the constant line from value 10 to value 20 on the PrimaryX axis.

constLine.EnableBeginEnd = true;
constLine.BeginValue = 10;
constLine.EndValue = 20;
constLine.BeginEndAxis = (int)StandardAxis.PrimaryX;

constLine.EnableBeginEnd = True
constLine.BeginValue = 10
constLine.EndValue = 20
constLine.BeginEndAxis = CType(StandardAxis.PrimaryX, Integer)

Each const line can contain text that denotes the line meaning. The following code snippet applies text to a const line:

By default the axis const line values are not included in the calculation of the axis range they are attached to. In case you need to make sure that the const line value is always included in the axis range you can use the IncludeInAxisRange property: