Summary of Daily Ranking Changes
The ranking index summarizes all ranking developments of all monitored products for all monitored keywords. If the index rises, the rankings have will havel improved rather than fallen for most of the keywords.
The horizontal line simply offers a reference point of the index level 30 days ago.
Different Index Evaluation of Different Ranking Changes
The calculation for the ranking index does not consider all ranking changes equally:
1. Ranking changes on the front pages are factored more strongly into the index trend than changes on pages further behind.
Product A's ranking rises for Keyword X from position 10 to 5. The ranking therefore increases by +5. At the same time Product B's ranking falls from position 50 to 55, so that the reanking decreases by -5. Even though the total index change would be 0, the index trend will nontheless show a minimal upwards movement, as a rise by 5 positions from 10 to 5 will lead to a larger visibility change than a loss by 5 positions when ranked 50.
2. Changes leading to a ranking on a new page are factored more strongly into the index trend.
If Product A's ranking rises from position 17 to 16 and therefore changes from page 2 to page 1, this will be factored more strongly into the index than a change from position 16 to 15 would have. This is because the visibility increases significantly more by appearing on page 1 compared to page 2, rather than than just moving a position on the same page.
Observation of Individual Product Rankings
If your ranking index has fallen strongly, it's probably worth going to "Rankings" and sorting the index collumn to see which products are affected the most:
For products whose organic visibility fell by a lot, the most frequent tactic would be to increase your PPC bids or/and budget for the respective campaign and its keywords.
On the product detail page (via far right column under Rankings), you can also identify the individual keywords whose rankings fell the most.