George - whenever you call plot like with
plot(grab_avg_time,grab_avg, '--*')
you are creating a new graphics object and so its data cannot be connected to data that has been plotted previously via other graphics objects. What you may want to do instead is to create one plot graphics object and update its data on each iteration of the while loop. For example, you would create the grpahics object as
hPlot = plot(NaN, NaN, '--*);
Then, in the while loop, you would update this object as
xdata = [get(hPlot,'XData') grab_avg_time]; % update the x-data
ydata = [get(hPlot,'YData') grab_avg]; % update the y-data
set(hPlot,'XData',xdata,'YData',data);
Note that instead of using a while loop, you could use a timer. See How to plot a real time signal with axes automatically updating for an example.
