If you could help that would be great!

The XAML line with visibility for the chart is as follows
<chartingToolkit:Chart Visibility="{Binding BrightnessValueList, Converter={StaticResource nonEmptyCollectionToVisibilityConverter}, UpdateSourceTrigger=PropertyChanged}" Height="218" Margin="0,0,20,127" x:Name="columnChart" Title="Star Brightness" VerticalAlignment="Bottom" Grid.Column="2">

My converter is called once when the application starts up and then not again. The converter is doing its job. I just need it called when items are added to the ObservableCollection :
this.BrightnessValueList.Add(new KeyValuePair<string, double>(focuserMotorStepPosition.ToString("00000"), starMaxBrightness));

BrightnessValueList is defined as follows:
public ObservableCollection<KeyValuePair<string, double>> BrightnessValueList {get; set; }

If more information is needed, just let me know. I thought the PropertyChanged was supposed to be raised with the converter being called when the "Count" property of the ObservableCollection changes (i.e. when adding an item in my case).

Thanks for looking!