[amibroker] PLOT OVERLAY AFL FOR RECESSIONS

[bistrader]

I would like to create an overlay afl that has recession periods in a color 
that can be selected via parameter with default of light gray.  I can come up 
with the beginning and ending dates for each period, but am not how to proceed. 
 Getting lost on how to best define (i.e., maybe 1 if in recession and 0 
otherwise) and how to best plot.  Help appreciated.