Adds a get-only property for the row called latest_sample which returns either the most recent sample or null if no samples are available.
Disallow multiple samples from the same sensor at the same instant.  Should be useful later for managing the cache of these samples.
Moves away from flot library to custom rendering in plot.js.  Allows filled area graphs.  Also copes with hi density displays to render to an oversize backing store to support the ppi of the canvas.