I faced the same problem too... Just rename one of the members of the cluster to X from Y and it will work fine...
Also, in case you don't want 1000s of plots mentioned in your graph and are content with two plots (or n, one for each color you want to portray,) just maintain the three arrays in shift register and instead of building a new array, append it to the previous one. Hope you understand what I mean by this.
