Is there a date/price matrix for Multi-City / open jaw flight booking comparison?

If you use google flights, pick round-trip, enter to/from cities then you get flight options but you also get a handy date/price matrix showing what the prices would be if you adjusted you start/return dates. I'm looking at booking a muti-city route EG ATL->MAD and return BCN->ATL and I'd ideally like a similar date/price matrix to find the cheapest set of dates to fly out / return because I'm flexible with dates. I've been checking out one-way pricing on each leg but I not sure that's completely accurate.