【WPF】自分で定義したプロパティにバインディングする

<Window x:Class="PropertyBindingLesson.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:my="clr-namespace:PropertyBindingLesson"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition></RowDefinition>
            <RowDefinition Height="24"></RowDefinition>
        </Grid.RowDefinitions>
        <my:MyUserControl MyScale="{Binding ElementName=slider,Path=Value}"></my:MyUserControl>
        <Slider Grid.Row="1" Name="slider" Minimum="1" Maximum="4" TickFrequency="0.1" />
    </Grid>
</Window>

<UserControl x:Class="PropertyBindingLesson.MyUserControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300" Background="AliceBlue">
    <Grid>
        <Canvas Name="canvas"></Canvas>
        <Button 
            Content="Button" 
            Height="23" 
            HorizontalAlignment="Left" 
            Margin="12,12,0,0" 
            Name="button1" 
            VerticalAlignment="Top" 
            Width="75" 
            Click="button1_Click" />
    </Grid>
</UserControl>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace PropertyBindingLesson
{
    /// <summary>
    /// MyUserControl.xaml の相互作用ロジック
    /// </summary>
    public partial class MyUserControl : UserControl
    {
        private ScaleTransform scaleTransform = new ScaleTransform(1.0d, 1.0d);

        // 依存プロパティ
        public static readonly DependencyProperty MyScaleProperty =
            DependencyProperty.Register(
                "MyScale",
                typeof(double),
                typeof(MyUserControl),
                new UIPropertyMetadata(
                    1.0d,
                    // PropertyChangedCallback
                    (d, e) =>
                    {
                        // プロパティ変更時の処理
                        // 新しい値をリソースのScaleTransformにセットする
                        (d as MyUserControl).scaleTransform.ScaleX = (double)e.NewValue;
                        (d as MyUserControl).scaleTransform.ScaleY = (double)e.NewValue;
                    })
            );

        /// <summary>
        /// 管理している全ての図形の倍率
        /// </summary>
        public double MyScale
        {
            get
            {
                return (double)GetValue(MyScaleProperty);
            }
            set
            {
                SetValue(MyScaleProperty, (double)value);
            }
        }

        /// <summary>
        /// コンストラクタ
        /// </summary>
        public MyUserControl()
        {
            InitializeComponent();
        }

        // ランダムな円を配置する
        private void button1_Click(object sender, RoutedEventArgs e)
        {
            Random rnd = new Random();
            
            int size = 10 + rnd.Next(100);
            Ellipse ellipse = new Ellipse(){
                Width = size,
                Height = size,
                Fill = new SolidColorBrush(
                    Color.FromArgb(
                        255,
                        (byte)rnd.Next(255),
                        (byte)rnd.Next(255),
                        (byte)rnd.Next(255)))
            };

            Canvas.SetTop( ellipse, rnd.Next((int)this.ActualHeight) );
            Canvas.SetLeft(ellipse, rnd.Next((int)this.ActualWidth) );

            ellipse.RenderTransform = scaleTransform;

            canvas.Children.Add(ellipse);
        }
    }
}