Hi Ján,
our "solution" is actually more like a hack, because on Android we don't have full control over what's shown in the calendar UI.
Since you have full control, you have much better options than we do.
I fully agree that it's not a good solution to show tasks just like events from "start" to "due". In contrast to events it's much easier to have a large number of ongoing tasks spamming your calendar view. But there is a value in seeing tasks in a calendar-like view, just have a look at Gantt charts. Also it's more convenient to see your entire agenda for a specific day in one view (including appointments and tasks).
one view for all data is useful, but mixing everything into incorrect interface is simply wrong ... Gantt chart is nice but it expects linear time (one time-line) and not time-line divided into X lines (month view). Using month view you will loose the visual advantage of this chart.
So, how about collapsing all on-going tasks to a single bar on top of each day that reads like "10 tasks, 2 starting, 4 due"
this sounds good, displaying 3 numbers (with text or icons) for each day is not a big problem, and also not breaks anything ... added to our todo list (but I cannot promise any deadline for this feature)
and add a button to expand the field to show all tasks for this day.
You could collapse all tasks into one bar or do that per collection using the appropriate list colors. That's just one option, I'm sure there are more.
this not sounds as good, because if we allow to "expand these tasks" in the calendar view (e.g. in a floating div), then users will ask why they cannot complete these todos, create new todos, etc. in the expanded list ... I don't want 2 different interfaces for the same thing