While this plugin was initially used to measure moon craters (together with the calculation plugins) it can easily be used to measure arbitrary lengths on pictures of your choice.
The moon plug-in (with the moon icon, shown on the left) only has one type of node: the image node.
Within the moon plug-in, a picture can be loaded into the image node by pressing the browse button. Once a picture has been loaded into the node it is resized adequately and shown within the node. Left-clicking into the picture produces a new layer with a new zoomed-in picture centred according to the pointer position. It is possible to switch between layers.
The calculation net plugin (graphical calculator for products and sums) contains several nodes:
the green input node
the yellow sum node
the blue product node
There are some edges in the Calculator, too. Depending on the nodes, the operational edges represent multiplicative (multiplication / division) or additive (addition / subtraction) operations. Here are the edges: