Okay, now I've reproduced it. It happens for either combo or bar chart.
The "Sorting" section provides as sort fields both the measure field "count(id_user)" and the dimension field "date_login". By default, it has the measure field as the first sort field. My first tactic was to turn it off, thinking that since sorting for only the dimension was turned on, that it would correctly sort by that field. For some reason, it does not. Changing from "Auto" to "numeric" or "alphabetic" sorting doesn't make any difference.
If I drag the date field to the first sort position, then it sorts correctly, using the "Auto" selection.