Draw line in billoard mode aligned with xz plane instead of facing camera

Hi, I'm trying to draw a line on the xz plane in billboard mode. The default behavior appears to rotate the billboard to face the camera (see red arrow in image). Instead, I want the line caps to lie flat on the xz plane so that the billboard is always facing up along the y axis (in line with the green arrow).

I'm happy to modify the code to achieve this if you could give me some pointers? Great asset btw :)