Thanks... I've looked at that previously, but the issue lies in that each layer doesn't have the same units or class breaks. That example looks like it requires you to set up the class breaks in the code, which won't work here. Some layers are integers, some are percentages, some are decimals, etc. And having the same units doesn't necessarily mean that the class breaks are the same.
So, for instance, Layer A may show the Asian Population for a neighborhood, which would be an percentage. Layer B might show Average Income for residents in neighborhood, which would be dollar amounts. I'd like to allow users to enable the Asian Population layer (which would be polygons) and then enable the Average Income on top of that, and it would show the circle symbols on top.
Looking at that example, it looks like I'd have to handle the class breaks depending on each layer, which I can't really hardcode.