Unless specifically designed to use multiple data tables, JMP Platforms, i.e Graph Builder requires the data to be graphed needs to reside in one data table. Jarmo's suggestion is a solution that will work because a Virtually joined data table looks like a single data table to a JMP Platform.
Unless the data tables are very large, I find that in JMP joining two data tables into one data table is such a simple and often used methodology, that I almost always just join the two data tables together.
Jim