<Grid x:Name="grid">
        <Grid.DataContext>
            <local:ViewModel/>
        </Grid.DataContext>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="80*"/>
            <ColumnDefinition Width="20*"/>
        </Grid.ColumnDefinitions>

        <chart:SfChart x:Name="chart">
            <chart:SfChart.PrimaryAxis>
                <chart:NumericalAxis x:Name="xAxis"/>
            </chart:SfChart.PrimaryAxis>

            <chart:SfChart.SecondaryAxis>
                <chart:NumericalAxis x:Name="yAxis" />
            </chart:SfChart.SecondaryAxis>

            <chart:ColumnSeries 
                ItemsSource="{Binding Collection}" 
                XBindingPath="XValue" 
                YBindingPath="YValue"/>
            <chart:SfChart.Annotations>
                <!--Text of rectangle annotation has been updated from TextBox-->
                <chart:RectangleAnnotation  
                    x:Name="annotation"
                    HorizontalTextAlignment="Stretch"
                    VerticalTextAlignment="Stretch"
                    X1="2.5"
                    Text="{Binding Source={x:Reference txtBlock},Path=Text}"
                    Y1="22"
                    ClipToBounds="True"/>
            </chart:SfChart.Annotations>
        </chart:SfChart>

        <StackPanel Orientation="Vertical" Grid.Column="1" HorizontalAlignment="Center" VerticalAlignment="Center" >
            <TextBlock Text="Change the annotation text" Margin="0,0,0,20" HorizontalAlignment="Center" VerticalAlignment="Center"/>
            <!--TextChanged event hook for updating its annotation X2,Y2 on every text changes-->
            <TextBox x:Name="txtBlock" TextChanged="txtBlock_TextChanged" />
        </StackPanel>
    </Grid>

private Size MeasuringString(string text)
        {
            if (string.IsNullOrEmpty(text))
            {
                return new Size(0, 0);
            }

            var TextBlock = new TextBlock()
            {
                Text = text,
                FontWeight = FontWeights.Bold,
            };

            TextBlock.Measure(new Size(Double.PositiveInfinity, Double.PositiveInfinity));
            return new Size(TextBlock.DesiredSize.Width, TextBlock.DesiredSize.Height);
        }

        private void UpdateAnnotation()
        {
            //Measured size from updated text of Textblock
            Size desireSize = MeasuringString(txtBlock.Text);
            var point1 = chart.ValueToPoint(xAxis, Convert.ToDouble(annotation.X1));
            var point2 = chart.ValueToPoint(yAxis, Convert.ToDouble(annotation.Y1));
            var point3 = point1 + desireSize.Width;
            var point4 = point2 + desireSize.Height;
            Point x2Points = new Point(point3, point4);
            var x2 = chart.PointToValue(chart.PrimaryAxis, x2Points);
            var y2 = chart.PointToValue(chart.SecondaryAxis, x2Points);
            annotation.X2 = x2;
            annotation.Y2 = y2;
        }


        private void txtBlock_TextChanged(object sender, TextChangedEventArgs e)
        {
            UpdateAnnotation();
        }