Collinear is when there is more than three points on plotted, and if they go on a same line.
The above four blue points are collinear, as they go on the same line.
To determine whether a series of points are collinear or not, you have to figure out the gradient by using the gradient formula: (Gradients (m))
It is sufficient to prove that the dots are collinear just by using two (do three just in case, though).
Although this isn’t really a merit/excellence part, this might be useful.