Hi Alex,

Thank you for contacting us. I have created a bug for this issue under the number of 36382 and I suppose it will be fixed in the next version of the control.

For now, here is a workaround:

<script type="text/javascript" >               
    function ShowTooltip(data, tooltipObj)
    {
        tooltipObj.style.zIndex = 200000;
    }
</script>

........

<igchart:UltraChart ID="UltraChart1" runat="server" ClientSideEvents-ClientOnShowTooltip="ShowTooltip" />

If you have some further question concerning the bug status, you could contact our support guys.