Since Python is right-exclusive, w.trace(seg_id) previously traced segment seg_id from iteration (w.iteration-1), rather than w.iteration.  Account for right-exclusivity.
@@ -562,7 +562,7 @@ class WIPI(WESTParallelTool):
key = []
except:
pass
- for iter in reversed(range(1, self.iteration)):
+ for iter in reversed(range(1, self.iteration+1)):
iter_group = self.data_reader.get_iter_group(iter)
particles = self.data_reader.data_manager.get_iter_summary(int(iter))['n_particles']
current['pcoord'].append(iter_group['pcoord'][seg_id, :, :])
