Hi,

If I understood correctly, you can change the number of the horizontal lines using the Unit property of the GridlineGroup element. 

For more information about the Unit property you can take a look here: http://help.infragistics.com/NetAdvantage/DV/2009.2/CLR3.5/?page=SL_DV_xamWebChart_Add_Grid_Lines_to_an_Axis.html

Please note the following code. Here the chart shows horizontal lines on every 2 units on the Y axis:

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  <Grid x:Name="LayoutRoot">

<igChart:XamWebChart x:Name="myChart" Width="300" Height="300">

            <igChart:XamWebChart.Axes>

                <igChart:Axis AxisType="PrimaryY"  AutoRange="True">

                    <igChart:Axis.MajorGridline>

                        <igChart:GridlineGroup Unit="2"/>

                    </igChart:Axis.MajorGridline>

                </igChart:Axis>

            </igChart:XamWebChart.Axes>

                <igChart:XamWebChart.Series>

                <igChart:Series ChartType="Column">

                    <igChart:Series.DataPoints>

                        <igChart:DataPoint Value="2" Label="First"/>

                        <igChart:DataPoint Value="5" Label="Second"/>

                    </igChart:Series.DataPoints>

                </igChart:Series>

            </igChart:XamWebChart.Series>

        </igChart:XamWebChart>

  </Grid>

This produces the following result:

Best regards,

Anatoli Iliev