I fixed it but I don’t know how so I couldn’t put the answer. And I found I can’t delete the post, either. … 
Then maybe the standard help is a starting point. Usually you select first the cells, then use Insert chart from menu and select appropiate settings in the assistant…
https://help.libreoffice.org/latest/lo/text/schart/main0000.html
maybe being able to delete clumsy posts would be a more sophisticated and neater, less labour intensive and less likely to invite superfluous comment way of organising things…