Every second, I would like to collect data (from some instrument) in one VI (collector.vi) and then add the point to a graph in another VI.
I've seen the "Dynamic Load" examples to use "Call SubVI by Reference." However, it seems like the data is simply passed once rather than continuously updated.
What is the simplest / best way to update a separate window (subvi) with periodic data?
You have a variety of choices beyond the dynamic call option you mentioned. You can also use a queues, notifiers and user events. Each has their benefits and their limitations. Check out the design patterns in the templates that ship with LV. From the file menu, select New... and in the resulting dialog, expand:
These templates show everything in one VI but the basic techniques are the same when the loops are in separate VIs. Pay particular attention to the Producer/Consumer Loop pattern.