what is the daily average of heart rate for patient 31880 in 01/last year?
select avg(chartevents.valuenum) from chartevents where chartevents.icustay_id in ( select icustays.icustay_id from icustays where icustays.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 31880 ) ) and chartevents.itemid in ( select d_items.itemid from d_items where d_items.label = 'heart rate' and d_items.linksto = 'chartevents' ) and datetime(chartevents.charttime,'start of year') = datetime(current_time,'start of year','-1 year') and strftime('%m',chartevents.charttime) = '01' group by strftime('%y-%m-%d',chartevents.charttime)