MainWindow.xaml

<Window x:Class="OxyPlotWpfTests.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:oxyPlotWpfTests="clr-namespace:OxyPlotWpfTests"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <oxyPlotWpfTests:MainWindowViewModel x:Key="MainWindowViewModel" />
        <oxyPlotWpfTests:CustomDataPointConverter x:Key="CustomDataPointConverter"/>
    </Window.Resources>
    <Grid x:Name="ThisGrid" DataContext="{Binding Source={StaticResource MainWindowViewModel}}">
        <oxy:PlotView
            Controller="{Binding Controller}"
            Model="{Binding PlotModel}">
            <oxy:PlotView.DefaultTrackerTemplate>
                <ControlTemplate>
                    <oxy:TrackerControl Position="{Binding Position}" LineExtents="{Binding PlotModel.PlotArea}">
                        <oxy:TrackerControl.Content>
                            <TextBlock>
                                <TextBlock.Text>
                                    <MultiBinding Converter="{StaticResource CustomDataPointConverter}">
                                        <MultiBinding.Bindings>
                                            <Binding  />
                                            <Binding ElementName="ThisGrid"
                                                     Path="DataContext.CustomTooltipProvider" />
                                        </MultiBinding.Bindings>
                                    </MultiBinding>
                                </TextBlock.Text>
                            </TextBlock>
                        </oxy:TrackerControl.Content>
                    </oxy:TrackerControl>
                </ControlTemplate>
            </oxy:PlotView.DefaultTrackerTemplate>
        </oxy:PlotView>
    </Grid>
</Window>

MainWindowViewModel.cs

using System;
using System.Diagnostics;
using System.Windows.Media;
using System.Windows.Threading;
using OxyPlot;
using OxyPlot.Axes;
using OxyPlot.Series;

namespace OxyPlotWpfTests
{
    using System;
    using System.Globalization;
    using System.Windows.Data;
    using OxyPlot;
    public class CustomDataPointConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            var hitResult = values[0] as TrackerHitResult;
            if (hitResult == null)
                return string.Empty;

            var tooltipProvider = values[1] as CustomTooltipProvider;
            if (tooltipProvider == null)
                return string.Empty;

            if (tooltipProvider.GetCustomTooltip == null)
                return hitResult.Text;

            var customTooltip = tooltipProvider.GetCustomTooltip(hitResult);
            if (string.IsNullOrWhiteSpace(customTooltip))
                return hitResult.Text;

            return hitResult.Text + Environment.NewLine + customTooltip;
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
    public class CustomTooltipProvider
    {
        public Func<TrackerHitResult, string> GetCustomTooltip { set; get; }
    }
    public class MainWindowViewModel
    {
        public CustomTooltipProvider CustomTooltipProvider { set; get; }
        public PlotModel PlotModel { get; set; }
        private readonly LineSeries _lineSeries1 = new LineSeries
        { 
            MarkerType = MarkerType.Circle,
            StrokeThickness = 2,
            MarkerSize = 3,
            Title = "Start",
        };
        private readonly LineSeries _lineSeries2 = new LineSeries { 
            MarkerType = MarkerType.Circle,
            Title = "End",
            StrokeThickness = 2,
            MarkerSize = 3,
        };
        private readonly LinearAxis _xAxis = new LinearAxis {
            Minimum = 0,
            MaximumPadding = 1,
            MinimumPadding = 1,
            Position = AxisPosition.Bottom,
            Title = "X axis",
            MajorGridlineStyle = LineStyle.Solid,
            MinorGridlineStyle = LineStyle.Dot
            };
        private readonly LinearAxis _yAxis = new LinearAxis { 
            Minimum = 0,
            MaximumPadding = 1,
            MinimumPadding = 1,
            Title = "Y axis",
            MajorGridlineStyle = LineStyle.Solid,
            MinorGridlineStyle = LineStyle.Dot,
        };
        private IPlotController _controller;
        private bool _haveNewPoints;
        private int _xMax;
        private int _yMax;
        public MainWindowViewModel()
        {
            initPlot();
            addPoints();
            updatePlot();
        }
        private void initPlot()
        {
            CustomTooltipProvider = new CustomTooltipProvider { GetCustomTooltip = (hitResult)=>{
                    var lineSeries = hitResult.Series as LineSeries;
                    var nearestPointIndex = hitResult.Index;
                    return "nearestPointIndex: " + nearestPointIndex;
                } 
            };
            PlotModel = new PlotModel
            {
                Title = "FFT",
                Subtitle = "Pan (right click and drag)/Zoom (Middle click and drag)/Reset (double-click)",
                Series = { _lineSeries1, _lineSeries2, },
                Axes = { _xAxis, _yAxis },
            };
            PlotModel.MouseDown += (sender, args) =>
            {
                if (args.ChangedButton == OxyMouseButton.Left && args.ClickCount == 2)
                {
                    foreach (var axis in PlotModel.Axes)
                        axis.Reset();

                    PlotModel.InvalidatePlot(false);
                }
            };
            foreach (var series in PlotModel.Series)
            {
                series.MouseDown += (s, e) =>
                {
                    if (e.ChangedButton == OxyMouseButton.Left)
                    {
                        PlotModel.Subtitle = "Index of nearest point in LineSeries: " + Math.Round(e.HitTestResult.Index);
                        PlotModel.InvalidatePlot(false);
                    }
                };
            }
        }
        private void addPoints()
        {
            var timer = new DispatcherTimer { Interval = TimeSpan.FromSeconds(1) };
            var rnd = new Random();
            var x = 1;
            _xMax = Math.Max(_xMax, x);
            timer.Tick += (sender, args) =>
            {
                foreach (LineSeries series in PlotModel.Series)
                {
                    var y = rnd.Next(100);
                    _yMax = Math.Max(_yMax, y);
                    series.Points.Add(new DataPoint(x, y));
                }
                _xMax = Math.Max(_xMax, ++x);
                _haveNewPoints = true;
            };
            timer.Start();
        }
        private void updatePlot()
        {
            CompositionTarget.Rendering += (sender, args) =>
            {
                if (_haveNewPoints)
                {
                    if (_yMax > 0 && _xMax > 0)
                    {
                        _yAxis.Maximum = _yMax + 3;
                        _xAxis.Maximum = _xMax + 1;
                    }

                    PlotModel.InvalidatePlot(false);

                    _haveNewPoints = false;
                }
            };
        }
        public IPlotController Controller
        {
            get
            {
                if (_controller == null)
                {
                    // show tracker with mouse move
                    _controller = new PlotController();
                    _controller.BindMouseEnter(PlotCommands.HoverPointsOnlyTrack);
                }
                return _controller;
            }
        }
    }
}