Error on installing mailgraph

Hello!

I have a problem installing on the 1.3.8.
I always get an error:

Quote

Plugin::Mailgraph::_createMailgraphPicture: Could not save png to '/var/www/imscp/gui/plugins/Mailgraph/tmp_graph/mailgraph_day.png'

There may be some kind of restriction with the folder.

Regards

thanks!