Hello kiencao42!
Welcome to our forum and thanks for contacting us with your question!
I'm not sure if I understood you correctly. Do you want to draw a horizontal line, from the beginning of the chart to its end?
If yes here you've got an example demo of how it can be done:
https://jsfiddle.net/BlackLabel/28jkvuno/
API references:
https://api.highcharts.com/highcharts/yAxis.plotLines
If no, please provide me some sketches or images how it should look like.
Best regards!