I finally got started working on the bubble counting circuit: a photointerrupter mounted across the bottom U-shape of an S-type airlock.  First I wired up the photointerrupter to the Spark core and verified that I could take readings from it.  I wrote some code to poll the photointerrupter rapidly, including some debouncing, and export the results through the same REST API call.  Now the results show up on the graph, as expected.  Next I tried mounting the photointerrupter on the airlock, but bubbles in the water don't consistently trigger the sensor.  I'm planning to experiment with various food dyes to see which are most IR opaque.  If that fails, I'll go back to the previous type of airlock, plus some black electrical tape to break the beam.  (Later I found out how to write interrupts on the Spark core, so I'll change that later.)