Also handle obs with zero off exposure nicely in the stacking
@@ -139,6 +139,7 @@ class ObservationStats(Stats):
gamma_rate = 0
bg_rate = 0
for stats in stats_list:
+ if stats.a_off > 0:
livetime += stats.livetime
n_on += stats.n_on
n_off += stats.n_off
