Setting agent_portrayal to None

[sanika-n]

I wanted to create a model with a property_layer, and I set agent_portryal to None as I was only interested in seeing how the property layer would change and not in the actual location of the agents themselves. However, the agents were still portrayed but using the default portrayal.
For context, I was using agent_portryal in functions under the matplotlib_components, implementing the following change in mpl_space_drawing.py fixed it for me...
Before

_scatter(ax, arguments, **kwargs)

After

if(agent_portrayal!=None):
      _scatter(ax, arguments, **kwargs)

I didn't post this in bugs as I wasn't entirely sure if it was one... I think I misunderstood the functionality of None and reverting to the default portrayal is the intended behavior, but if that is the case, I think it would be useful to have the option of not representing the agents at all as well...

[quaquel]

This is related to #2638 and #2640.

I need to think about the best way forward to address this. Yes, we can add a bunch of if statements as per #2640. However, I personally am inclined to see this as an architectural problem. At the moment, we don't distinguish carefully between drawing the space (e.g., the grid structure), the drawing of properties of the space (i.e., property layer portrayal), and the drawing of agents within the space. These are effectively all collapsed into a single draw_space method. I am inclined to redesign the API and explicitly separate these three things slightly. If you pass the same axes (in the case of matplotlib), you can draw them on top of each other, but if you pass separate axes, you can then also portray them more cleanly.

[Sahil-Chhoker]

I think the drawing of space and property layers is quite intuitive with this design. We draw the space and define the portrayal of its properties along with it. However, I can't say the same about current agent portrayal, I do recall finding it very difficult at first to understand how the agent_portrayal method magically colors all the agents accordingly, until I read the API documentation. I’m not sure if these thoughts are relevant to this discussion, but I am strongly in favor of redesigning the API to separate the drawing of grids and agents.

[quaquel]

My experience is slightly different. I find, in particular, the nested dict stuff with property layers challenging to remember and use. Moreover, it is quite restrictive.