Linear interpolation with one point
You can use linear interpolation for points to annotate a moving object:
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Before you start, select the Points.
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Linear interpolation works only with one point, so you need to set Number of pointsto 1.
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After that select the Track. 
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Click Trackto enter the drawing mode left-click to create a point and after that shape will be automatically completed. 
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Move forward a few frames and move the point to the desired position, this way you will create a keyframe and intermediate frames will be drawn automatically. You can work with this object as with an interpolated track: you can hide it using the Outside, move around keyframes, etc. 
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This way you’ll get linear interpolation using the Points.