Using a Python script, how do you change Marker Layers on a Map Chart?

I have two marker layers on a map.  One to select by counties and then it returns On-Demand data back to the map.  I want to build a Python script that will select the next Marker Layer without people having to know how to use the Layers Control to switch the layers.  I just want them to click a button and then be able to select from the On-Demand data.  I have been unsuccessful in finding any helpful examples on this site and on the Internet in general.   Any Python examples will be greatly appreciated.  Thank you. 

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