Is there a way to apply a global template to all tooltips? I get my datapoints from a data source and I don't understand how I would apply the tooltip formatting to every single one of them.

In this case you can use:

<Grid.Resources>

<Style TargetType="ToolTip">

<Setter Property="FontSize" Value="40" />

<Setter Property="Template">

<Setter.Value>

<ControlTemplate TargetType="ToolTip">

<Border BorderBrush="Black" BorderThickness="2">

<ContentPresenter Content="{TemplateBinding Content}" />

</Border>

</ControlTemplate>

</Setter.Value>

</Setter>

</Style>

<igCA:XamChart Name="xamChart1">

...

How do you change the font size of tooltip in code?

For

M As Integer = 1 To 12
series.DataPoints.Add(
New DataPoint() With {.Label = Maaned(M), .Value = Data_Fravær(i, M), ToolTip = String.Format("{0:N2}", Data_Fravær(i, M))})
Next