Markers of graph (conditional charting)

Hello everyone,

I have a question on conditional charting.
I am using scatter xy graph straight line with marker.
Let me ask in this way: Now I have a few data. After plotting using scatter xy graph, I find the average, and I will be draw a horizontal straight line on the plot area.
So is there anyway to customize my scatter xy graph so that it shows a different marker whenever the value is more than (average*2)?
And is there a way to customize the marker so it also will show another different marker when there are 6 or more points in the graph is increasing/decreasing?
p.s. my data is dynamic, and the graph fluctuate a lot.