Peter,
As far as I know, you cannot programatically chage the size of the cluster output in the Array to Cluster vi. However, you may not need to convert to a cluster for charting. The chart will take a 2D array and plot mutiple lines. See the attached example which uses Reshape Array to form the data for a chart.
The only hitch is that it erases the plot when you change the number of values to plot. This may or may not be important to you.
Tim