If you need it just for charting (you asked about charting software):
- Sierra Chart can chart both ets and synthetics - basic version 26 usd/month, but you still need some data feed (you can have it for free with some brokers)
- Tradingview is very good option if you don't need real time data it's for free (you have both stocks and futures), but with free version you can chart synthetic spreads only in daily resolution, if you need intraday it costs 29 usd/month or you can use some "spread" indicator to have intraday spreads charts for free. For real time data you need to pay for data or have it as your charting platform with some of brokers (it uses CQG data). For Stocks real time data is rather cheap here
- IB TWS can chart synthetic spreads too
- you can also use Ninjatrader for charting for free, but you still need some data feed for that and import spread indicator to make it work
Iw will be good if you precise what for you need it. And do you need realtime data or some delayed is ok, or "end of day" is ok. Do you need availability to execute trades or not. E.t.c.