XAML:
                <Path x:Name="Mouth" Stroke="Black" StrokeThickness="3">
                    <Path.Data>

                        <MultiBinding Converter="{StaticResource MouthConverter}">
                            <Binding ElementName="HeadYPosSlider" Path="Value" />
                            <Binding ElementName="MouthHeightSlider" Path="Value" />
                            <Binding ElementName="MouthWidthSlider" Path="Value" />
                            <Binding ElementName="myCanvas" Path="Width" />
                            <Binding ElementName="MouthHappinessSlider" Path="Value" />
                        </MultiBinding>

                    </Path.Data>
                </Path>

C#:
    public class MouthConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            PathGeometry pg = new PathGeometry();
            pg.Figures = new PathFigureCollection()
            {
                new PathFigure()
            };


            var hyp = values[0];
            var mhs = values[1];
            var mws = values[2];
            var mcw = values[3];
            var hap = values[4];

            double HeadYPos = (double)hyp,
                MouthHeight = (double)mhs,
                MouthWidth = (double)mws,
                CanvasWidth = (double)mcw,
                Happiness = (double)hap;


            pg.Figures[0].StartPoint = new Point(((CanvasWidth / 2) - MouthWidth), (HeadYPos + MouthHeight));
            ArcSegment point2 = new ArcSegment();
            point2.Point = new Point(((CanvasWidth / 2) + MouthWidth), (HeadYPos + MouthHeight));
            point2.Size = new Size(Happiness,50);
            pg.Figures[0].Segments.Add(point2);

            return pg;
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }