Do not interpolate over missing data

[jameeters]

All those straight lines are interpolations over the time when Central's meters were offline. Since there's no actual data for that time, there should be no lines.

[huss]

We agreed that missing data should not have lines. As a general question, we need to decide how we know when data is missing - what is the time that has elapsed before we do this.

We also discussed bar type graphics. It seems in this case it would be best if we could put another box around the area with missing data. That way we don't have to muck with the bars (which would be hard) but the user would know that data is missing so impacted bars would be clear. We would need some label so the user understands what this means. There is an open questions about how easy/hard this will be in Chartjs.

Finally, since we plan to bring on other graphing types, we should have some API for determining missing points that will work for lines, charts, radar, etc.