System.Windows.Shapes.Path incorrect behavior

After doing some research on subject I didn't find anything, so I'm sorry if the same question was already asked.

Task: make a colored track-line on Canvas after cursor, when the left mouse button is pressed (like brush in Paint).

Problem: I think using System.Windows.Shapes.Path is the best approach to doing this task. Code below works fine, except for one thing: if you try to move your cursor then change direction to the opposite (e.g. the value on X-axis increases, then decreases, but the value on Y-axis, stays constant), you will get an unexpected part of Line, corresponding to the previous direction.

I'm sorry for the tangled description of my problem, but I hope you will get it.

To make it easier for you to reproduce it on your machine I'm adding the solution.

Please, point out for my mistake if I did one!

Link to the application: http://ge.tt/99aSgyo/v/0?c

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
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 WpfApplication3
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private Boolean Inserting;
        private Path path;
        private Boolean isFirstPoint;

        public MainWindow()
        {
            InitializeComponent();
            LolCanvas.IsHitTestVisible = true;
        }
    
        private void Canvas_MouseMove(object sender, MouseEventArgs e)
        {
            if (Inserting)
            {
                Point p = Mouse.GetPosition(LolCanvas);
                if (isFirstPoint)
                {
                    PathFigure myPathFigure = new PathFigure();
                    myPathFigure.StartPoint = new Point(p.X + 5, p.Y + 5);
                    myPathFigure.Segments = new PathSegmentCollection();
                    (path.Data as PathGeometry).Figures.Add(myPathFigure);
                    isFirstPoint = false;
                }
                else
                {
                    LineSegment myLineSegment = new LineSegment();
                    myLineSegment.Point = new Point(p.X + 5, p.Y + 5);
                    (path.Data as PathGeometry).Figures[0].Segments.Add(myLineSegment);
                }
            }
        }

        private void Canvas_MouseDown(object sender, MouseButtonEventArgs e)
        {
            Inserting = true;
            path = new Path();
            path.Stroke = new SolidColorBrush(Colors.Red);
            path.StrokeThickness = 50;
            path.Data = new PathGeometry();
            (path.Data as PathGeometry).Figures = new PathFigureCollection();
            LolCanvas.Children.Add(path);
            isFirstPoint = true;
        }

        private void Canvas_MouseUp(object sender, MouseButtonEventArgs e)
        {
            Inserting = false;
        }
    }
}

<Window x:Class="WpfApplication3.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525"><Canvas x:Name="LolCanvas" MouseMove="Canvas_MouseMove" MouseDown="Canvas_MouseDown" MouseUp="Canvas_MouseUp" Background="Black"></Canvas></Window>