Feature request: plot of scalar properties as function of environment/frame index

[Luthaf]

This would allow to look at the evolution of a property in a dataset, in particular trajectories from simulations.

This is already achievable by adding a index property, and then using the map to show property = f(index), but maybe this could be made easier.

A first step we could take to make it easier is to automatically add such index property to the dataset.

After that, we could use a separate widget to show the plots, maybe reusing the work done in #78?