works like a charm

This is what I was looking for. I can easily update charts each month by using Google Drive for the spreadsheet. Adjust the numbers in the spreadsheet and paste a new url from the spreadsheet into this charts plugin, and the chart is updated. No csv needed. And the tutorials are very clear.
I combined it with a plugin to show certain pages only to certain logged in users (customers) so only the customer (and admin) can see their own chart. For their convenience I created a login on the frontend of the customer’s own page.