Hello Graham,

Yes, the behavior you've described is expected. If the mouse is positioned over a vertical line, the tooltip may change since it's bordering two different values/intervals.

The tooltip only takes the x axis into consideration when deciding which data value to be displayed.

This causes the tooltip to display the same regardless if the mouse is on the left or right side of the data point along the horizontal line of a given segment in the step line series. And the tooltip will not change when moving along the y axis.

Having the chart pick up where the mouse is along the y axis as well as horizontal line to manipulate which value to display is considered a feature request. You can suggest new product ideas for future versions (or vote for existing ones) at <http://ideas.infragistics.com>.

There are many benefits to submitting a product idea:

Direct communication with our product management team regarding your product idea.
Notifications whenever new information regarding your idea becomes available.
Ability to vote on your favorite product ideas to let us know which ones are the most important to you. You will have ten votes for this and can change which ideas you are voting for at any time.
Allow you to shape the future of our products by requesting new controls and products altogether.
You and other developers can discuss existing product ideas with members of our Product Management team.

Steps to create your idea:

1. Log into the Infragistics Product Idea site at http://ideas.infragistics.com (creating a new login if needed).

2. Navigate to the product / platform channel of your choice (e.g. WPF, Windows Forms, ASP.NET, HTML5 / Ignite UI, iOS / NucliOS, etc.)

3. Add your product idea and be sure to be specific and provide as much detail as possible.

   - Explain the context in which a feature would be used, why it is needed, why it can't be accomplished today, and who would benefit from it. You can even add screenshots to build a stronger case. Remember that for your suggestion to be successful, you need other members of the community to vote for it. Be convincing!

   - Include a link to this thread in your idea so product management will be able to look back at this case.

The Product Idea site puts you in the driver's seat and allows you to track the progress of your ideas at any time, see how many votes it got, read comments from other developers in the community, and see if someone from the product team has additional questions for you.

Alternatively you can remove default tooltips and add a custom ItemTooltipLayer which, by default, appears by snapping to each datapoint the mouse is closest to. You just need to add it to the series.

eg.

            var itemToolTipLayerSeries = new ItemToolTipLayer();
            itemToolTipLayerSeries.TargetSeries = series1;

            this.xamDataChart.Series.Add(itemToolTipLayerSeries);

Let me know if you have any questions regarding this matter.