Hi Qingjun,
Thanks for using Syncfusion Products.
You can able to achieve your requirement using ChartAreaPaint Event feature in Syncfusion Chart Control.
Please take a look at the sample attached.
http://websamples.syncfusion.com/samples/Chart.Windows/F61676/main.htmThe above sample exhibit how to draw a custom grid lines inside the chart Area.
Please let me know if this helps.
Regards,
Rashidha.