Note that when you rotate it, the width and height remain relative.
Let's say you want to create a callout that starts next to a bullet point and stretches to touch another object on the page.
Create a box a couple of pixels wide with a stroke at the top edge, set the transform origin to 0/0 (the top left corner), position it next to the bullet, set starting and ending keyframes for the width , go to the second keyframe, then drag the right edge to the desired width. At this point you can rotate it to align with the desired endpoint. At this point you can change the width and the line will follow the angle you have set.
You can combine this with imported art, such as an arrowhead. Before performing the rotation, align the arrowhead with the right end of the stroke and animate the position of the arrowhead to move in sync with the width. Now select the rectangle and the arrowhead and group them. Select the group object, set the horizontal transformation point to 0, and nudge the vertical point so that it aligns with the stroke. At this point you can adjust the rotation.
Callout.hype.zip (11.8 KB)