Hi, i just checked and yes it looks like a bug, i don't check for any exception in my library in case you only want to use 2 series, what you can do for the meantime is set the data for series3 to 0 and also set the color of series3 to colors.transparent, that way it will not show up in your graph.
I will try to fix this as soon as i get a chance.
Thanks,
Walter