We’re all here to learn and share!
The easiest way that I can think of is to drag a plane into the ‘HIERARCHY’ module and create a timeline for it. You can find out more information about timelines here!
I’d start the timeline with the plane invisible (or as small as your starting size is) and then end it with the target size using the scale property. Studio does this really cool thing where the bits in-between are generally animated for you, so all you need for a simple extending line is a start value and a finish value!
You could of course do this with a .png, it’s totally up to you, though depending on the size of your line it might get a bit fuzzy - have a play around.
For the plane to be diagonal, you could rotate it so it’s facing the correct angle.
Have a great day,