A z-score demonstrates a value with respect to an average (assuming the data is normally distributed) so a z-score of 0 means that a given data point is perfectly average compared to the whole sample while a z-score of 2 indicates that a given value is well-above average. Similarly, a negative z-score implies a value is below average. A really good description of z-scores is posted at https://www.youtube.com/watch?v=16vAjsnazEM.

This graph was meant to show how each year's frequency of terrorist attacks faired compared to the average number of attacks (which for 2000-2020 was 6,660). If my graph was more clear it would have demonstrated that the year 2004 had the lowest frequency and was the furthest away from the mean. While the years 2011 and 2012 were closest to the mean on the low and high end respectively. Year 2014 shows how the total number of attacks that year was furthest away from the mean (16,960 Events). Hopefully, this clears up what my intent was even though I missed the mark.