Let me try to add some more color:

1. Create an event handler for the DataItemOver event of the chart. The easiest way to do this is to go to the form design tab, click on the lightening bolt in the properties window, scroll down to DataItemOver, and double click it. This will create a new method for the event handler.
2. In that method, paste the code that mtanska provided:

   	int oldLayerIndex = layerIndex;
           if (e.LayerID == "Layer One")
           	layerIndex = 0;
           else if (e.LayerID == "Layer Two")
           	layerIndex = 1;
           else if (e.LayerID == "Layer Three")
           	layerIndex = 2;
   
           if (oldLayerIndex != layerIndex)
                   CustomizeLabels(layerIndex);

3. Add a private variable to your form called layerIndex (this will store which layer we're showing tooltips for right now).
4. Make sure all your layers have the appropriate names. You can do this by setting the Key property of the ChartLayerAppearance to things like "Layer One", "Layer Two", etc in mtanska's example.
5. Add the CustomizeLabels function that mtanska provided above. NOTE: you'll have to make sure that you don't already have other custom renders defined, otherwise the function above will wipe them out. For that reason, I recommend you use the function below instead. Rather than wiping out all things in the hashtable, it only eliminates the tooltips:

   private void CustomizeTooltips(int layerIndex)
   {
   chart.Tooltips.Format = TooltipStyle.Custom;
   chart.Tooltips.FormatString = "<TOOLTIP>";
   Hashtable tooltips = chart.LabelHash;
   if (tooltips["TOOLTIP"] != null)
   tooltips.Remove("TOOLTIP");tooltips.Add("TOOLTIP", new RenderTooltip(layerIndex));
   chart.LabelHash = tooltips;
   }

6. Create a new class that implements the IRenderLabel interface. In this class you'll need a constructor to store the layer ID you're passing in, and a ToString method that implements the logic you want:

   using System;
   using System.Collections;
   using System.Text;
   using Infragistics.UltraChart.Shared.Styles;
   using Infragistics.UltraChart.Resources;
   class RenderTooltip : IRenderLabel
   {private int layerID;
   
   public RenderTooltip(int layerID)
   	{this.layerID = layerID;
   	}
   	public string ToString(Hashtable context)
   	{
   		// Add your logic here
   		if (layerID == 0)
   return context["DATA_VALUE"].ToString();
   		else
   return ...;
   	}
   }

Hope that helps!
 
Thanks mtanska for your posts - they got me to the solution I needed!
Yash.

 i dont find the data item over event handler in the events list of the ultrachart..

any solns?

 
Are you sure you're using Infragistics UltraChart for Windows Forms?  Is the dll you're using named Infragistics2.Win.UltraWinChart.vX.x.dll ?

 Want to set two different scales on the same y axis for a column chart

i.e.,

 with a minumum range of 0 to a max of 10.. i wanna set 0 - 5 with an interval of 1 and from 5 to 10 wit 2 as the inteval..

any solutions???

 To do this, you need to create a Composite Chart with 2 ColumnChart layers, each of which uses a different Y-axis.