Except for the code segments that render the magnifying glass (as circle or path), this is essentially a copy of Mike Bostock’s Fisheye Distortion example.
The motivation for this was largely aesthetical but also a bit functional. When using the Fisheye as an aid to viewing large datasets, I often find myself experimenting with the degree and radius of the distortion. In such cases, I find it helpful to see the exact reach of the distortion and then decide the best configuration for a particular dataset.