Thank you for your help, Elizabeth. I believe that this is probably the answer I need, but I'm having difficulty translating it from VB. Are you able to help me with that? Thanks again!

Hello again,

When I attempt to implement this code verbatim, I see no change in my chart gridlines. The only change I had to make was converting the igChart reference to igWebChart in the xaml. If you have any insight into why this might be the case, I would greatly appreciate it. Thanks again!

I'm sorry, I should probably be more clear - my gridlines don't appear dotted no matter what I try with this approach. The xamChart to xamWebChart is just the only modification I made after copying and pasting. I receive no errors, and the gridlines just display as normal.

Hmm... works for me...

Are you sure you are assigning the grid line style?

                    <igChart:Axis.MajorGridline>
                        <igChart:GridlineGroup Stroke="Red" StrokeThickness="3" GridlineStyle="{StaticResource LineStyle}" />
                    </igChart:Axis.MajorGridline>

Here is the full code of my sample in c#:

The xaml:

<UserControl x:Class="SilverlightApplication109.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
    xmlns:igChart="clr-namespace:Infragistics.Silverlight.Chart;assembly=InfragisticsSL4.Controls.Charts.XamWebChart.v10.3" 
    xmlns:local="clr-namespace:SilverlightApplication109" mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">
    <UserControl.Resources>
        <Style x:Key="LineStyle" TargetType="igChart:Mark">
            <Setter Property="StrokeThickness" Value=".5"/>
            <Setter Property="Stroke" Value="#FFE6EFF7"/>
            <Setter Property="Fill" Value="#FFE6EFF7"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="igChart:Mark">
                        <Grid x:Name="parentGrid">
                            <Grid.Resources>
                                <local:DoublingConverter x:Key="DoublingConverter"/>
                            </Grid.Resources>
                            <local:BoundsRectangle x:Name="Bounds" />

                            <Path UseLayoutRounding="True" Stroke="{TemplateBinding Stroke}" 
                                  StrokeThickness="{Binding Path=StrokeThickness, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource DoublingConverter}}"
                                  StrokeDashArray="3" 
                                  Data="{Binding ElementName=Bounds, Path=PathData, Mode=OneWay}">
                            </Path>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </UserControl.Resources>

    <Grid x:Name="LayoutRoot">
        <igChart:XamWebChart>
            <igChart:XamWebChart.Axes>
                <igChart:Axis AxisType="PrimaryY">
                    <igChart:Axis.MajorGridline>
                        <igChart:GridlineGroup Stroke="Red" StrokeThickness="3" GridlineStyle="{StaticResource LineStyle}" />
                    </igChart:Axis.MajorGridline>
                </igChart:Axis>
            </igChart:XamWebChart.Axes>

            <igChart:XamWebChart.Series>
                <igChart:Series>
                    <igChart:Series.DataPoints>
                        <igChart:DataPoint Value="1" />
                        <igChart:DataPoint Value="2" />
                        <igChart:DataPoint Value="3" />
                        <igChart:DataPoint Value="4" />
                        <igChart:DataPoint Value="5" />
                    </igChart:Series.DataPoints>
                </igChart:Series>
            </igChart:XamWebChart.Series>
        </igChart:XamWebChart>
    </Grid>

</UserControl>

And the code behind:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Media;

namespace SilverlightApplication109
{

    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
        }
    }


    public class BoundsRectangle : Panel
    {
        public static DependencyProperty PathDataProperty =
        DependencyProperty.Register(
        "PathData",
        typeof(GeometryGroup),
        typeof(BoundsRectangle),
        new PropertyMetadata(PathDataPropertyChanged));

        public BoundsRectangle()
        {
            this.SizeChanged += OnSizeChanged;
        }

        public GeometryGroup PathData
        {
            get { return (GeometryGroup)GetValue(PathDataProperty); }
            set { SetValue(PathDataProperty, value); }
        }


        private void OnSizeChanged(
            object sender,
            SizeChangedEventArgs args)
        {
            double newWidth = args.NewSize.Width;

            GeometryGroup newData = new GeometryGroup();
            LineGeometry top = new LineGeometry();
            top.StartPoint = new Point(0, 0);
            top.EndPoint = new Point(newWidth, 0);

            newData.Children.Add(top);

            PathData = newData;
        }

        private static void PathDataPropertyChanged(
            DependencyObject sender,
        DependencyPropertyChangedEventArgs args)
        {
        }
    }

    public class DoublingConverter : IValueConverter
    {
        public object Convert(
            object value,
            Type targetType,
            object parameter,
            System.Globalization.CultureInfo culture)
        {
            if (value != null && value is double)
            {
                return (double)value * 2.0;
            }

            return null;
        }

        public object ConvertBack(
            object value, 
            Type targetType, 
            object parameter,
        System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}

-Graham

Thank you Graham! I'm not sure what the difference was between your code and mine, but everything works now.