Improve time series plots to not display NAs or empty cells

[sgwinder]

We would like to be able to include NAs in the data tables (mmm, mmmir, www, wwwir) and have those be missing data points on the time series plots. Currently we are able to make this work for the social media platforms and on-site data by leaving the cells "empty" in the csv's. But this doesn't seem to be working for the modeled results (jjmm), which are still being displayed as "zeros" when the data is missing.

So, a minimum first step would be to treat the modeled results in the same way as the social media data. But a longer term solution would be to allow NAs in the csvs to be reflected as missing data on the time series plots.

[sgwinder]

Just a note that this is still a high priority issue!

[sgwinder]

Continues to be an issue - see patch in trails-viz-data commit bfa1b1