In part of your question you call the control a graph, and in other
parts a chart. The answer depends a bit on which you are showing and
hiding. Graphs only show the most recent data written to them, charts
retain and show history.
Neither the graph or chart will have much overhead receiving data while
hidden. On the otherhand, it isn't zero either since they will at least
be doing a data copy so they can draw with correct data when shown.
It will be easiest to send the data all the time since otherwise, you
will need to send fresh data when showing the graph. If dealing charts,
not only will you need to update the current data, but the history will
be old. You will need to clear or write the history.
If it doesn't cause a problem, write to them always, but if you can
measure a problem or have spare time on your hands, add code to make the
update conditional and deal with the data when being shown.