Multiple activites on same day not displayed

When i make a custom plot in the compare category and have multiple activities on the same day. There is only a single point being plotted in a scatterplot for that day even if there are multiple activities. I guess this is because the scatter plot only expects to plot one point for each day, rather than for each activity.

If I move one activity to a different day both activities are displayed. If I enable a filter in the custom plot i that removes the activity that is displayed, then the other is displayed instead. So clearly the scatterplot can only plot one point per day.

If there is an aggregate function somewhere that is choosing for instance to maximum of the two i could not find it.

That‘s no bug, that’s per design. As you already recognized, only one dot per day is possible.
For custom activity fields, you can choose the aggregate from the description tab:

If you want to change the aggregate from a built-in variable, you also have to first create a custom field, fill it with the built-in variable on the script tab and choose your own aggregate on the description tab.