You do not need a converter for this, use the .ToString() method at the property.
public string GraphPenWidthValue { get { return this.GraphPenWidth.ToString(); } }
Anyway here is a Standart String Value Converter:
[ValueConversion(typeof(object), typeof(string))]
public class StringConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return value == null ? null : value.ToString();
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}