I guess your X-Axis is a category axis?
Hiding the label is basically the same deal, you need somehow to decide which datapoints (I mean the labels of which datapoints) to hide, make a property for that in your ViewModel and make a Style for the AxisLabel control Binding to that...
The mark itself is a bit more complicated, there is the MajorTickMarkStyle-Property on your Axis but it is the same for every label... I'm not quite sure how I would approach that.
But maybe it's okay to leave the marks itself.
I've evaluated the usability another diagram which hid many of the label + marks and often the users tended to think that there's not more data available, so it should made be visible somehow that there's more data.