Charts are created from the sidebar. Once they have created a chart, administrators will find a button below the chart allowing you to add it to your chart library.
They are managed from the admin page, where they can be modified or deleted.
Each chart can be assigned to a particular combination of customer and location (or alternatively, you can assign it to all users). This helps control who has access to them, as not all charts will be relevant to everyone.
Note that it is not possible to change the content of a chart once it has been created - you can only change the name and the customer/location assignment. If you want to replace a chart, you'll need to delete it and create a new one with the same name.