I have a set of 2D lines (contour lines from a collection of images), and would like to somehow represent them with an "average" and some measure of spread (se figure below). What could be a good way to achieve this? It will only be used for illustrative purposes, so the simpler the better.

[![2D lines summary](https://i.stack.imgur.com/OMGMq.png)](https://i.stack.imgur.com/OMGMq.png)