Figure out how best to plot NULL values

[melange396]

They currently show up as "0" (zero), as can be seen in an example that looks at NSSP values for Manhattan (FIPS 36061). (you can verify that the values in question are actually null and not 0 by looking at the API response your browser's network debugging tool)

This is potentially very misleading.

Another option is to "ignore" null values in a series, treating them as though they dont exist , which will show up as no plotted line at all (or a gap in a plotted line if real values precede and follow the nulls).

Other suggestions are welcome.

[melange396]

With #104, we now treat null values as if they effectively dont exist. We can leave it like this, or we can make a toggle to display nulls as zeros, or we could do some other type of visual indicator.