MVVM Binding in Ribbon.Group

<ribbon:Tab x:Class="Sato.ChartDocument.Ribbons.ChartDocumentRibbonTab"
            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
            xmlns:ribbon="http://schemas.actiprosoftware.com/winfx/xaml/ribbon"
            xmlns:rv="clr-namespace:Sato.ChartDocument.Ribbons"
            xmlns:vm="clr-namespace:Sato.ChartDocument.ViewModels"
            Label="Chart Properties">
    <ribbon:Tab.DataContext>
        <vm:ChartDocumentViewModel/>
    </ribbon:Tab.DataContext>
    <ribbon:Group>
        <StackPanel Orientation="Horizontal">
            <Label Content="Chart Title" />
            <TextBox Text="{Binding Path=ChartTitle, Mode=TwoWay}" />
        </StackPanel>
    </ribbon:Group>
</ribbon:Tab>

namespace Sato.ChartDocument.ViewModels
{
    using Microsoft.Practices.Prism.ViewModel;

    /// <summary>
    /// Description of chart document view model
    /// </summary>
    public class ChartDocumentViewModel : NotificationObject
    {
        private string chartTitle = "Tytuł ViewModel";

        /// <summary>
        /// Gets or sets the chart title.
        /// </summary>
        /// <value>
        /// The chart title.
        /// </value>
        public string ChartTitle
        {
            get { return chartTitle; }
            set
            {
                if (chartTitle != value)
                {
                    chartTitle = value;
                    RaisePropertyChanged(() => this.ChartTitle);
                }
            }
        }
    }
}