How can we improve the update performance for an XamDataChart using the following?

a. 15 ScatterLineSeries with 1 new data point added per second and data in an ObservableCollection
b.  10,000 - 40,000 data points per line
c.  No data point markers, no fancy line styles
d. Data is added to each series at random times during the 1 second interval (i.e., comes from a serial device) and also comes in a random order.  
e. These updates come in on 1 thread per data series and all updates to the observable collection are routed through the graph UI thread
f. X axis is time the data was collected (i.e, cannot use LineSeries)
g. The xamdataChart built-in zoom mini-map widget is located in the lower right hand corner. 

This graph functions with good speed for about 1 hour and then becomes unresponssive.  For example, if you zoom in when the graph has good performance, wait, and then keyboard up/down/left/right after 1 hour, it take 20-30 seconds to shift the zoomed in are by 1 keystroke.

The CPU after 1 hour was a flat 25% of a 4 core CPU machine.   Our VS 2010 profiling showed that 75% or more of the time was spent in a UI rendering code (maybe due to refreshing the graph 15 times each second).