What inspired this wish list request? Being able to calculate a distance is great, but then you are like "oh, wouldn't it be great to keep the ruler here to show people!"
What is the improvement you would like to see? a way to keep the graph builder ruler (or any graphic ruler, i suppose) permanent on the graph so it can be saved to show people in journals or PowerPoint, etc...
Why is this idea important? As stated above, to showcase what you can see with the ruler, there is no way to 'freeze' it so it sticks around. If I want to show people I need to either do it live OR create a graphics script to build it ... the ruler is much cleaner and pretty than what is easy to do with a graphics script.