XV/GIMP/etc: Can't open display

[Dave Horsfall]

MacBook Pro (13-inch, Mid 2010), saddled with High Sierra 10.13.6 (as far 
as it will go).

Back when I was running Sierra these tools worked just fine; after 
upgrading to HS (and rebuilding MacPorts that took almost a day) things 
went pear-shaped:

    mackie:~ dave$ xv
    xv: Can't open display
    mackie:~ dave$ gimp
    Cannot open display: 
    mackie:~ dave$ echo $DISPLAY

    mackie:~ dave$ 

Is this something I forgot to do during the upgrade?  I set $DISPLAY to 
":0" (an old Unix trick) to no effect, so I guess it's using a named pipe 
instead but I cannot find it; poking around doesn't seem to uncover 
anything applicable to this, and I certainly won't trust any "advice" on 
StackOverflow.

Where has my 70yo brain screwed up?

Thanks.

-- Dave