line mark question about group

It was my mistake.Because my data is read from a CSV file, its actual structure is as follows:

[ {date: “2024-01-01”, year: 2024, cost: 10796}, {date: “2024-01-02”, year: 2024, cost: 11403}, … ]

However, the result I want is Figure 2, but the actual output from the code is Figure 1. Could you please help me make it display like Figure 2?