Something like this (sorry for the multiple edits - there's no preview of comments in Splunk Answers):
index=rl_ivr avg_speed_answer=*
| eval nn=tonumber(replace(replace(avg_speed_answer,"\.",""),",","."))
| eval media_giorno = call_offered * nn
| eval mythreshold="4"
| timechart span=3mon first(mythreshold) sum(call_offered) as total_call sum(media_giorno) as total_avg_speed_answer
| eval row_average=(total_call * total_avg_speed_answer)
| eval sla10 = total_avg_speed_answer / total_call
| eval sla10=round(sla10,2)
| rangemap field=sla10 red=6-100 default=green
| fields _time,sla10
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