Hi Will, so this question is in regards to the interval lesson. The diagram you are looking at is simply showing you the distance between the 8 (counting the root, twice) notes of the major scale. I labeled them on alternate strings to help visually represent the distance on the staff (standard notation) and tablature.
C to D is not shown stacked, because they are on the same string; And the closest D would have been the 5th fret of string 1, which would have been an octave further. The distance would be the same if you played it all along the third string.
Further along in the course, you will learn multiple positions for playing the C major scale.