Support plotting agent-level variables

[EwoutH]

I tried it quickly, but it would be great to be able to plot agent-level variables.

For example, in the Forest Fire model, I might want to create a plot in which I can view how many trees are Burned Out, Fine and On Fire. These are currently only available as agent level variables, and it would be great to be able to plot these.

[Corvince]

Agent level variables are definitely something that must be included, but it's not clear how they should look like. In your example I would say it's actually a model level variables, because we would have to aggregate the values by adding them. Other times we probably want to have mean values. But again, they become model level variables. A true agent level plot would either include all agents or a subset

[EwoutH]

I built this once for the old visualisation: mesa/mesa#1145.

A question is what needs to go in mesa-interactive and what in Mesa itself. Maybe we can add this to the dev meeting agenda.