If I understand correctly, right now the Analysis plugin and the histograms use only the mean of each activity for displaying how much time (or distance) one spends monthly or weekly in the respective pace or heart rate zones.
However, for interval workouts the mean pace is roughly the same or only slightly faster than the pace of low to medium intensity runs.
So this analysis does not really provide a good metric on how much time one spends in the higher heart rate and pace zones.
It would be great to make this Analysis more useful by displaying the actual times spent in the pace zones.
Of course this requires more looping through activities...
P.S.: Runalyze is awesome.