Indicate holes in data

[kamocat]

Because we're logging both the start and end time for each point, we can track holes in the data by looking for a large gap between the end time of one entry and the start of the next.
Here's some ideas to display that in D3
https://bocoup.com/blog/showing-missing-data-in-line-charts

Note that the logging script assumes if the value hasn't changed, there is no gap in the data. This may not hold true, in which case we'll need to actually define the expected sample rate of each topic.