Reiner discovered that units were assumed to be unchanging, in append. The first day had "seconds since 2012-001" the second was "seconds since 2012-002". I was thinking this was supported, but apparently not since the code assumes the units don't change and don't check to see if they do.
I'm making it so append checks for a units change.
We just noticed that fill value is dropped when they are not the same. See https://sourceforge.net/p/autoplot/bugs/2409/