How to calculate max/min scales on a scatter plot

#1
Good morning!

I have 3 log scatter plots that I want to establish smooth maximum and minimum lines. What is the usual mathematical method to do that? (Image and excel file links below.)

The black lines on the scatter plot images are hand drawn. The third scatter plot is especially tricky and not amenable to a moving average plus stddev because of the data clumping. Note: This is time series data so new data constantly comes in. In other words, I cannot just use the whole data population in one shot.

Any ideas would be greatly appreciated.

Excel File: https://dl.dropboxusercontent.com/u/44057708/Three Scatters.xls
Image at: https://dl.dropboxusercontent.com/u/44057708/ThreeScatters.jpg
 
#2
I've generated a working solution and just wanted to post it for future generations ;)

Someone pointed out that because the data is not really fall under "standard error" due to the gaps, that medians +/- stddev would not really work. I have confirmed this with many days of attempts.

Therefore, the other typical solution, as far as I can tell, is called rank-type smoothing or moving quantiles; with damping of the end result. Basically, using a sample size of 100, I take an average of the 10 largest items for the maximum and an average of the 25 smallest items for the minimum. I then dampen the changes to 10% for smoothing. This gives me a smooth enough maximum and minimum line to use as a 0-1 scale.

Hope this helps!