Installation brought an error with some text as:
"Could not install ... file ... to /var/www/imscp/gui/plugins/Mailgraph/tmp_graph/mailgraph...png"
Solution:
Added directory manually, changed owner and group.
Restarted (forced) Installation.
Installation completed.
Found 13 files inside tmp_graph directory afterwards. Have to wait, 'till the graphs are populated ;-).