How should I interpret (BTC/CNY) / (BTC/USD)?
Regarding your question before the update:

1) Yes, they have the same units ("how many units of CNY you need to buy one USD", either directly or through a bitcoin intermediary). That is the whole point of arbitrage. 

2) It seems that "the global forex market"  and "the bitcoin market" seem separate to you. If by the former you mean big investment banks, then maybe you are right, as there seem to be no indication that big investment banks are systematically engaging in Bitcoin trading (at least I found no evidence of it; please correct me if I'm wrong). However, there are tons of bitcoin-oriented banks and brokers doing transactions on currency pairs, and I'm not sure these should not be counted as part of the global forex market. 

Having said this, I think the simplest interpretation of the lines is that of a currency pair, and that of the currency pair being arbitraged through bitcoin. 

3) The green line refers to how many units of CNY you need to buy one unit of USD. Or equivalently, how many units of CNY you get for one unit of USD. If the purple line is above (below) the green line, then you can get more (less) units of CNY by (1) selling USD through bitcoin and (2) buying CNY with those bitcoins. This means your interpretation in the update is correct. If you have USD, sell them to BTC, and buy CNY (purple line), and then exchange these directly to USD. As the green line is below the purple one, you need less CNY to buy one USD, which is the same to say that you get more USD for the same amount of CNY.

So, there seem to be arbitrage opportunities, at least in a zero transaction cost market.