Connect the K-line data of the current main contract and the previous expired main contract on the same K-line diagram.

The main contract is the current contract of a certain variety in the futures market. The main contract is to connect these main contracts in chronological order to form a continuous K-line chart. In this way, we can observe and analyze the historical trend of coking coal futures, including the price and volume of previous main contracts and the current trend of main contracts.