How to make Rectangle size unchanged after axis changing

Question

I want to make Rectangle that would not scale size after axis changing. After changing X And Y minimum or maximum, OxyPlot scaling Rectangle. And i don't know the "new" minimum and maximum of X and Y Axes. Is it simple way to do this?

using GalaSoft.MvvmLight.Command;
using OxyPlot;
using OxyPlot.Annotations;
using OxyPlot.Axes;
using OxyPlot.Series;
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 WpfApplication22
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new PlotRectangle();
    }
}
public class PlotRectangle
{
    public ICommand ComboCommand { get; set; }
    public PlotModel PlotModel { get; set; }
    public LinearAxis XAxis { get; set; }
    public LinearAxis YAxis { get; set; }
    public LineSeries LineSeries { get; set; }
    public RectangleAnnotation Rect {get;set;}
    public List<string> Items { get; set; }
    public PlotRectangle()
    {
        ComboCommand = new RelayCommand(()=>ChangeAxis());
        PlotModel = new PlotModel();
        Rect = new RectangleAnnotation();
        LineSeries = new LineSeries();
        Items = new List<string>();
        Items.Add("5");
        Items.Add("10");
        Items.Add("15");

        Rect.MinimumX = 10;
        Rect.MaximumX = 20;
        Rect.MinimumY = 1;
        Rect.MaximumY = 10;
        XAxis = new LinearAxis
        {
            Position=AxisPosition.Bottom,
        };
        YAxis = new LinearAxis
        {
            Position = AxisPosition.Left
        };
        PlotModel.Axes.Add(XAxis);
        PlotModel.Axes.Add(YAxis);
        PlotModel.Series.Add(LineSeries);
        PlotModel.Annotations.Add(Rect);
    }
    public void ChangeAxis()
    {
        Random rnd1 = new Random();
        Random rnd2 = new Random();
        Random rnd3 = new Random();
        Random rnd4 = new Random();
        PlotModel.Axes[1] = new LinearAxis
        {
            Minimum=rnd1.Next(1,10),
            Maximum=rnd2.Next(15,50)
        };
        PlotModel.Axes[0] = new LinearAxis
        {
            Minimum = rnd3.Next(1, 10),
            Maximum = rnd4.Next(15, 50)
        };
        PlotModel.InvalidatePlot(true);
    }
}
}

And Xaml look like this

<Window x:Class="WpfApplication22.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:oxy="http://oxyplot.org/wpf"
    xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
    Title="MainWindow" Height="350" Width="525">
<StackPanel Orientation="Horizontal">
    <ComboBox Height="20" VerticalAlignment="Top" SelectedIndex="0" ItemsSource="{Binding Items}">
        <i:Interaction.Triggers>
        <i:EventTrigger EventName="SelectionChanged">
                <i:InvokeCommandAction Command="{Binding ComboCommand}"></i:InvokeCommandAction>
        </i:EventTrigger>
        </i:Interaction.Triggers>
    </ComboBox>
    <Grid>

<oxy:Plot Model="{Binding PlotModel}" Width="490"></oxy:Plot>
</Grid>
</StackPanel>

I need to draw on Screen, but how?