Changing the x-axis to the Time of Night in the Long Term Visibility Indicator (lovely feature!) produces a black band during the period that in reality there is no astronomical darkness anymore at my site, which would mean all day darkness.

Fixed.
Bonus: a. The graphic background is now computed by a thread, so there isn't a 10 second delay during which tha app freezes. b. Fixed some other problems with circumpolar objects, etc.