Hi,

I was able to achieve this for a single series using the following style:

 <Style TargetType="igChart:LegendItem" x:Name="LegendItemStyle1">

            <Setter Property="Foreground" Value="#FF617583"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="igChart:LegendItem">
                        <Grid x:Name="RootElement">
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="Auto"/>
                                <ColumnDefinition Width="*"/>
                            </Grid.ColumnDefinitions>
                            <Rectangle RadiusX="3" RadiusY="3" x:Name="LegendItemIcon" Grid.Column="0"
                                   Width="13" Height="13" 
                                   Fill="{TemplateBinding Fill}" 
                                   Stroke="{TemplateBinding Stroke}" 
                                   StrokeThickness="{TemplateBinding StrokeThickness}"
                                   />
                            <TextBlock x:Name="LegendItemText" Margin="5,0,0,0"  Width="Auto" Height="Auto" Grid.Column="1"
                                   Text="{TemplateBinding Text}"
                                   Foreground="Black">
                                   <ToolTipService.ToolTip>
                                       <TextBlock Width="300" Height="Auto" Text="{Binding ElementName=Chart1, Path=Series[0].Label}"/>
                                   </ToolTipService.ToolTip>
                             </TextBlock>
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>

However, the limitation here is that you can set the style only the Legend using the LegendItemStyle property, which means you're limited to only one series for this to work properly.  I basically templated the Tooltip to be a textblock and bound its text to the label of the first series in the chart.  Simply binding it to the LegendItem text won't work, because the LegendItem text actually becomes the truncated version, as opposed to the series label (which you would expect).  Perhaps you'll be able to further implement this to use for more than one series.

Hope this is a start,

Marisa