I think it's the average.
hi socgt blue line is actually baseline average which compares selected query data with historical data (default is actually automatic time range which is past five interval )
charts are actually selected period data.
Thank you @kmc for the explanation . I just want to ask one more question that is in some stack distribution graphs the blue line was not present what is the reason for that.
Also on y-axis the total number events collected is plotted ?