Hi Francisco,
Thanks for contacting Syncfusion support.
We are able to achieve your requirement Line with average in ColumnSeries by adding a new Line series at the average YBindingPath value of Column series. We have prepared a sample and it is available for download in below location. In this sample, we have added new Line series with Numerical XAxis which axis visibility is hidden.
Please let us know if you need any further assistance.
Regards,
Gobi M