WinUI Chart provides the support to display the needed information from its model of populated items source along with the Tooltip UI customization with the help of the TooltipTemplate in the chart series as shown in the following code example.
Here, display both country's name and population details in the tooltip. By default, it shows the corresponding y-axis value of that segment.
XAML
<chart:SfCartesianChart Header="Population Growth">
<chart:SfCartesianChart.DataContext>
<local:ViewModel/>
</chart:SfCartesianChart.DataContext>
<chart:SfCartesianChart.Resources>
<ResourceDictionary>
<DataTemplate x:Key="tooltipTemplate">
<StackPanel Orientation="Horizontal">
<!--Template element has DataContext as its Segment named Item. From it, access the corresponding Model-->
<TextBlock FontFamily="Segoe UI" Foreground="White">
<Run Text="{Binding Item.Country}"/>
<Run Text=":"/>
<Run Text="{Binding Item.Population}"/>
</TextBlock>
</StackPanel>
</DataTemplate>
</ResourceDictionary>
</chart:SfCartesianChart.Resources>
<chart:SfCartesianChart.XAxes>
<chart:CategoryAxis/>
</chart:SfCartesianChart.XAxes>
<chart:SfCartesianChart.YAxes>
<chart:NumericalAxis/>
</chart:SfCartesianChart.YAxes>
<chart:ColumnSeries ItemsSource="{Binding Data}"
XBindingPath="Country" YBindingPath="Population"
EnableTooltip="True"
TooltipTemplate="{StaticResource tooltipTemplate}">
</chart:ColumnSeries>
</chart:SfCartesianChart>
KB article - How to display more data in the tooltip in WinUI Chart (SfCartesianChart)?