Add legend to your chart and set its LegendText property .
<legends>
<asp:legend backcolor="Transparent" alignment="Center" docking="Bottom" font="Trebuchet MS, 8.25pt, style=Bold" istextautofit="False" name="Default" legendstyle="Column" forecolor="255, 255, 255" xmlns:asp="#unknown"></asp:legend>
</legends>
Chart1.Series[0].LegendText = "#VALX";