Passing Values between user Controls in Silverlight Appplication

I am created silverlight application ,in that application two user control in 1st usercontrol i put combobox and as per selected that combobox value bind chart in another usercontrol . i am try pass that combo box value to another usercontrol .follow my two user control code

1) combobox.xaml

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:System="clr-namespace:System;assembly=mscorlib" xmlns:ee="http://schemas.microsoft.com/expression/2010/effects"
    mc:Ignorable="d"
              xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
    d:DesignHeight="300" d:DesignWidth="400" >
 
       
           
            
            
           
               
                   
                   
                   
                   
               
           
           
           
           

2)ChartUserControl.xaml

    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
             xmlns:telerikChart="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls.Charting"
             xmlns:telerikCharting="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls.Charting"
 x:Name="parent"
    mc:Ignorable="d" >
 
 
  
  
 
 
 
         Foreground="{StaticResource text}" AxisForeground="{StaticResource text}"  FontFamily="Segoe UI Light" Width="400" Margin="0,0,0,0">
          
       

 
 

ChartUserControl.xaml.cs

  public ChartUserControl()
        {
            InitializeComponent();
         
            radChart.DefaultView.ChartLegend.Visibility = Visibility.Collapsed;
            PieSeriesDefinition piechart = new PieSeriesDefinition();
            radChart.DefaultSeriesDefinition = piechart;
            MainViewModel mvm = new MainViewModel();
            DataContext = mvm;
            radChart.ItemsSource = mvm.customers;
        }

here  MainViewModel  code

 public class MainViewModel
    {

           private Service1Client client;

           public MainViewModel()
           {
               client = new Service1Client();
               client.TypeSolutionAsync();
               client.GetGridDetailAsync();
               client.SolutionAsync();
               client.GetGridDetailCompleted += new EventHandler(client_GetGridDetailCompleted);
               client.SolutionCompleted += new EventHandler(client_SolutionCompleted);
               this.solution = new ObservableCollection();
               this.customers = new ObservableCollection();
               this.Gridobjects = new ObservableCollection();
               client.TypeSolutionCompleted += new EventHandler(client_TypeSolutionCompleted);
           }
           public void client_TypeSolutionCompleted(object o, TypeSolutionCompletedEventArgs e)
           {
               if (e.Error == null && e.Result != null)
               {
                   foreach (displaydata c in e.Result)
                   {
                       this.customers.Add(c);
                   }
               }
           }
           public void client_GetGridDetailCompleted(object o, GetGridDetailCompletedEventArgs e)
           {
               if (e.Error == null && e.Result != null)
               {
                   foreach (GridData c in e.Result)
                   {
                       this.Gridobjects.Add(c);
                   }
               }

           }
        public void client_SolutionCompleted(object o, SolutionCompletedEventArgs e)
           {
               if (e.Error == null && e.Result != null)
               {
                   foreach (SolutionData c in e.Result)
                   {
                       this.solution.Add(c);

                   }
               }
           }

           public ObservableCollection customers
           {
               get;
               set;
           }
           public ObservableCollection Gridobjects
           {
               get;
               set;

           }
        public ObservableCollection solution
           {
               get;
               set;
           }
    }

please help to solve this problem