The SOVs on the graph are daily averages calculated by the steps described as follows -
Every hour, the system receives scraped results from Amazon for every hourly tracked keyword. When we receive a keyword, we calculate its hourly SOV. Say a brand appeared in 2 results out of 10, its SOV for that hour would be 20%. As we receive more results, we keep aggregating them over the day. If the brand receives no impressions in the following hour, its aggregate SOV for that day would be (assuming a total of 20 results in the following hour): (0+2)/(20+10) = 6.67%