Hi Christmas Fantasy,
We have reviewed your query and created a simple sample by including a few empty data points at the beginning of the spline area series. In our end, the UI did not display a long line starting from the beginning of the axis in the spline area chart.
We’ve shared this sample for your reference. However, if the long line still appears in your case, kindly modify the shared sample to reproduce the issue that showing the line starting from the axis and send it back to us. Additionally, please provide more details about the specific scenario in which this behavior occurs. This will help us assist you more effectively.
Screenshot:

Regards,
Mugunthan.
Attachment:
fr_196624_a528985b.zip