If you set xAxis label align to left, last label sometime overlaps. Here is an example: http://jsfiddle.net/vbpDS/1409/ (try to resize result area)
Forum thread: http://highslide.com/forum/viewtopic.php?f=9&t=9143
There's no logic in Highcharts to prevent overlapping. The user must set a tickInterval high enough to prevent it, slant the labels or apply staggerLines...
Is there a way to access the xAxis labels from the load event to check for collisions after rendering? e.g. Check if the x position of a label is within x + width of the previous label.
If this is possible then I could choose to re-render my chart with stagger or slant enabled.
Overlapping avoidance was added in Highcharts 3.0.8.