Variant of interval scheduling with varying task durations

I am probably just missing the correct term for my problem to find the solution but here it goes:

I have a set of tasks with a given duration and an interval for each task in which it has to be completed. I now need to find start times for the tasks, so that

1. each tasks is completed within its interval

Sketch: • Welcome to Computer Science! What have you tried? Where did you get stuck? Above all, what is your question? Also, even though this question looks like not complicated, can you add a url or reference to in the question? That will help people to answer your question faster and better. Oct 26 '18 at 1:31
• I have added a different image that maybe explains the problem better. I need to shift the colored bars in positions so that no colored bar overlaps with another AND each colored bar is within its intrerval (black bars). Oct 26 '18 at 11:06
• Can you tell us whether this is an open problem, tricky problem or an entry-level problem or just a complete new problem created by you? Context is just too important. That is why I am going to ask it again, can you add a url or reference? By the way, your image is really wonderful. Oct 26 '18 at 12:52
• This seems an NP-complete problem by a reduction from 3-Partition, as suggested by this paper, but it does not give a detailed proof. Oct 26 '18 at 13:03

Given an instance $$x_1,x_2,\ldots,x_{3m}$$ of 3-Partition, let the duration in your problem be $$[0, mB+m]$$ where $$B=\sum_{i=1}^{3m} x_i/m$$. For each element $$x_i$$, there is a task with duration $$x_i$$. The intervals for these tasks are all $$[0, mB+m]$$. In addition, there are additionally $$m$$ tasks with duration $$1$$, and their intervals are respectively $$[B, B+1], [2B+1, 2B+2],\ldots,[mB+m-1,mB+m]$$. Note the positions of these additional tasks are fixed.
Since we can assume $$B/4 for all $$i$$ in 3-Partition, we can see there is a valid partition if and only if there is a valid arrangement in your problem.