[xy-chart] consider deltaX in findClosestDatums, fix tickLabelProps bug in <*Axis />

[williaster]

This PR does the following

🐛 Bug Fix

• Fixes a bug where tickLabelProps is not used when passed in either <XAxis /> or <YAxis />. This prop enables per-tick styles so is importanté!

• Improves the logic for finding closest datums across series when eventTrigger=container by considering deltaX from mouseX in addition to deltaY from mouseY. If deltaX is not considered, the "closest" datum in the y dimension might belong to a series whose nearest x value is quite far from mouseX (if the other data points are null, for instance):

Before

After

[coveralls]

Coverage increased (+0.04%) to 84.457% when pulling 3a594de on chris--bugz into 2a72bc7 on master.