O(n) exact algorithm to calculate the new stdev of an set of points after the insertion of a new value given the number of points, old stdev and old mean. The idea to make this algorithm came from the Messari Subgraphs Discord
The key insight for this method is that new_std*(new_len-1)-old_std*(old_len-1)=(new_val-new_mean)(new_val-old_mean)