===== MainPage.xaml =====

<UserControl x:Class="SilverlightApplication3.MainPage"
    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:oxy="clr-namespace:OxyPlot.Silverlight;assembly=OxyPlot.Silverlight"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

    <Grid x:Name="LayoutRoot" Background="White">
        <oxy:Plot x:Name="FFTplotter"/>
    </Grid>
</UserControl>

===== MainPage.xaml.cs =====

using System;
using OxyPlot;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.ComponentModel;
using System.Windows.Controls;
using System.Collections.Generic;

namespace SilverlightApplication3
{
    public partial class MainPage : UserControl, INotifyPropertyChanged
    {
        public MainPage()
        {
            InitializeComponent();
            FFTplotter.Model = new PlotModel();
            SetPlotData();

            SelectedValues = new ObservableCollection<int>();

            FFTplotter.Model.MouseDown += MouseDown;
            FFTplotter.Model.MouseUp += MouseUp;
            FFTplotter.Model.MouseMove += MouseMoved;
        }

        private void SetPlotData()
        {
            // Set the type of lineseries and the kind of visualization
            var ls = new LineSeries();

            var ps = new List<DataPoint>();
            Random random = new Random();
            var min = 0;
            var max = 100;

            for (int x = 0; x < 1000; x++)
            {
                var dataPoint = 1;
                ps.Add(new DataPoint(x, random.Next(min, max)));
            }

            ls.ItemsSource = ps;

            FFTplotter.Model.Series.Clear();
            FFTplotter.Model.Series.Add(ls);
            FFTplotter.Model.InvalidatePlot(true);
        }

        private ObservableCollection<int> _selectedFrequencies;
        public ObservableCollection<int> SelectedValues
        {
            get { return _selectedFrequencies; }
            set
            {
                if (value == null) return;
                _selectedFrequencies = value;
                NotifyPropertyChanged("SelectedFrequencies");
            }
        }

        private bool _mousedown;
        private void MouseDown(object sender, OxyMouseEventArgs e)
        {
            _mousedown = true;
            e.Handled = true;
        }

        private void MouseUp(object sender, OxyMouseEventArgs e)
        {
            _mousedown = false;
            RenderSelectedValue(e);
            e.Handled = true;
        }

        private void MouseMoved(object sender, OxyMouseEventArgs e)
        {
            if (_mousedown)
            {
                RenderSelectedValue(e);
                e.Handled = true;
            }
        }

        private ObservableCollection<Annotation> _selectedFrequencylines;
        private static bool RenderingLines;

        private void RenderSelectedValue(OxyMouseEventArgs e)
        {
            if (!RenderingLines)
            {
                RenderingLines = true;

                Axis xaxis, yaxis;
                FFTplotter.Model.GetAxesFromPoint(e.Position, out xaxis, out yaxis);
                if (xaxis == null) return;
                double x = xaxis.InverseTransform(e.Position.X);
                var a = new LineAnnotation
                {
                    Type = LineAnnotationType.Vertical,
                    X = x,
                    Color = OxyColors.Gray,
                    LineStyle = LineStyle.Solid,
                };

                // Add the new line
                FFTplotter.Model.Annotations.Add(a);
                FFTplotter.Model.Title = FFTplotter.Model.Annotations.Count + " annotations added";

                FFTplotter.Model.InvalidatePlot(false);

                RenderingLines = false;
            }
            else
            {
            }
        }

        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        /// <summary>
        ///   Generic NotifyPropertyChanged function
        /// </summary>
        /// <param name="propertyname"> Name of the property that has changed </param>
        private void NotifyPropertyChanged(string propertyname)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyname));
            }
        }
        #endregion
    }
}