I created a workspace and added 2 charts. One is a chart type of Sum and the other one is of Custom with a custom Chart Source. Both are set as Public but the second chart is only visible to the System Manager.
I tried to add Role Permissions and User Permissions with no luck. The only way I was able to make it visible was by editing the Frappe core code to disable permission verification, which proves it is a permission issue.
But I could not figure out how to fix this without modifying the Frappe code.