Here's a quick fix for your xaml style to ensure you don't get the "invisible foreground" tooltip text for your Chart DataPoint styles in the Cosmo theme. The problem currently looks something like this:

To fix this, all you need to do is to add a Foreground color definition to override the one that’s currently set on the parent chart control, since the Chart tooltip is picking up the LabelTextBrush from the overall foreground color of the parent chart control.

So in your ChartDataPoint style you want to do something similar to this:

 

<Style x:Key="ColumnDataPointStyle1" TargetType="Charting:ColumnDataPoint">

       <Setter Property="Background" Value="Orange"/>        

       <Setter Property="BorderBrush" Value="Black"/>

       <Setter Property="BorderThickness" Value="1"/>

       <Setter Property="IsTabStop" Value="False"/>

       <Setter Property="Template">

              <Setter.Value>

                     <ControlTemplate TargetType="Charting:ColumnDataPoint">

                           <Border x:Name="Root" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Opacity="0">

                                  <ToolTipService.ToolTip>

                                         <ContentControl Content="{TemplateBinding FormattedDependentValue}" Foreground="White"/>

                                  </ToolTipService.ToolTip>......

 

 So you will get something like this:

 Hope this helps!