Planar Surface Component won't allow negative Y value; Rectangle will - ideas?

Hi,

So if I add some basic plan geometry using GH, and manipulate it in the same way, two similar components behave differently which is confusing. 

I am creating an area indicator bar, with a graded shading preview, so the easiest way (I think) is to use a shaded Planar Surface component. But it won't draw a negative Y value. Compared to the Rectangle component, which will. 

Have a look at the image and this definition (everything else removed including shading).

I want the surface to draw below the origin with negative numbers, and above with positive numbers, that's the idea; as the rectangle does. 

Thanks for any comments and tips. Welcome alternative ways to do the same, and of course, I am not guaranteeing I am not doing something v dumb already.