Help with Histogram Excel

create a resource call it lumpsum make it labor and the default 8 hr/day

make sure that calculate costs from units is NOT checked.

filter for all of the activites that you want resources on

then highlight them all right click and select Assign then resource and select lumpsum or the one you just created

this will automatically multiply the duration by 8 to give you units ie. 5 days = 5*8=40 hours so thats 1 man on that activity for 5 days 

you change the unist to whatver you want or you can export the Budgeted Labor units to excel then change the units in excel 

and then import the correct units back into p6

after that select view show on bottom resource usage profile and you histogram is there

what schedule package are you using  ?