don't plot masked spectra values

[sbailey]

Raga Pucha identified that Nightwatch is always plotting fibers 2664 and 2665 with huge outliers like this:

These are fibers hitting bad CCD columns with mostly ivar=0 (not not mask>0 for some reason...). Update spectra plotting to not show ivar=0 or mask!=0 values (or maybe show them differently, e.g. in light gray and don't let them drive the vertical scale).