Hi famproject,

The axis tickmarks are part of the same container that houses the axis labels and this container is specifically placed relative to the chart.  So there's no built in way to tell the tickmarks they need to extend upwards, outside the bounds of its container.  What you can do instead is add another axis but set the label location to "InsideBottom"  This will cause the container to be placed inside the chart area at the bottom rather than the default outside bottom.  This will also flip the tickmarks and labels so the tickmarks are at the bottom.

If you are using a NumericXAxis you will have to make sure that your second axis is bound to the minimum, maximum and interval properties of the first axis so they stay in sync.  If you are using a CategoryXAxis you need to make sure the second axis is bound to the same ItemsSource as the first axis.  For CategoryDateTimeXAxis, not only do you need the same ItemsSource, you also need the same minimum, maximum and interval values as the main axis.

I have attached a sample that demonstrates how this should work.  I suggest submitting a product idea for this if you wish to see this functionality built into the product. (http://ideas.infragistics.com)