There are two charts that show up for Stores Trend. The Added to Store by Day show you a day by day chart of when a product was added to store. This is a good way to see the breakdown of the actual days that the product was being added to stores around the world.
The Added to Store Trend chart on the other hand is a great way to see the "trend" of a product at a glance. if the chart is still going up, it means new stores are adding this product, while a flat chart shows that no new stores are adding this product.