Because the main contract is the contract with the largest volume, which contract has the largest volume, the K-line on the futures is the K-line chart of this contract.
For example, it's April, and if the XX 1407 contract has the largest transaction volume now, the continuous chart shows the K line of the July contract. May and August have the largest contract turnover, so the continuous chart shows the K-line chart in August. And so on.