function called for report_mig_mult objects renames columns replaces nulls, and calculates reports with time period larger than day
fun_report_mig_interannual(dat, annee = NULL, timesplit = NULL)
a data frame with mean, max, and min calculated for each timesplit
a data frame with columns ("bjo_annee","bjo_jour","bjo_labelquantite","bjo_valeur")
The year to exclude from the historical series (it will be plotted against the historical series)
"week" "2 week" "month" as provided to seq.POSIXt, default NULL