After looking at the chart again, there appears to be a negative average receiver separation for Fields, which doesn’t make any sense and means that there’s a baseline that is not provided; and further, what then constitutes an “open receiver”? I’d imagine that this measurement is something like this: The separation of the receiver a QB throws to less the baseline, divided by the sum of all of the receivers’ separation, expressed as a percentage. The y-axis is probably a piecewise function for each time a receiver had more than the baseline amount of yards over total number of throws. So, if the baseline is 2 yards, and we have a set of 5 throws expressed in yards = {1, 3, 12, 4, 15}, and the % of throws to open receivers is the horizontal line 80%, because 1 < 2, which is 0, and the rest of the throws are greater than 2, so (0+1+1+1+1)/5 = 80%.