Show rotation axis in the image viewer

[dagewa]

I'm working on the VMXi thaumatin data (dials.data get vmxi_thaumtin). Just after import I see this image:

I see the backstop shadow is vertical and I wonder if I have to mask it, because maybe the rotation axis is horizontal. Unfortunately I have no way to see the rotation axis direction.

However, this feature is available in dials.image_viewer:

and looking at that I see that the axis is aligned to the shadow and there is no need for me to mask.

Please add the option to show the rotation axis in DUI2, so I don't have to go back to the dials.image_viewer.