Align vertices in a line NOT in axes [duplicate]

Question

This question already has an answer here:

• Straighten edges not on axis 5 answers

So here is a picture:

Here i have 3 verts. I want to align them in diagonal, centering on middle vertex.

There is a solution tho - creating an edge and setting it as a custom transform orientation, but it seems to be overcomplicated. What if i need to do this on multiple vertex groups?

May be there is another way to straighten verts in a line, that is not aligned with axes?

UPD: actually above situation can be solved by scaling along normal Z axis.

But what if i have this:

Here i want verts to be aligned in a line between first and last verts. But transforming by normal orientation does not help here.

UPD: tried making an edge between these two verts, but its orientation appeared unsuitable for scaling (like on image).

UPD: used lower face as transform orientation, but result was not as expected. And what if i have no suitable faces, oriented appropriately?

Please advice, what am i doing wrong here?

Answer 1

It appeared that the problem is easily solvable by creating the edge between two extreme verts by F and then subdividing that edge, so it has the corresponding number of verts. Then simply slide the verts of the edge with double G with enabled snapping to vertex, so every new verts are aligned with the verts that need to be straigtened. Lastly i managed to get perfect straight edge of verts by merging them!

Thanks Mr Zak for the hint!

Also i've found a nice plugin shipped with blender - Loop Tools. It has a tool named "Flatten". It also performs fine in this situation.