The HP 7475A wakes up with hardware handshaking enabled: DTR starts high and goes low when the internal 1 KB buffer has less than 80 bytes remaining. The plotter also supports XON/XOFF handshaking, a sad software thing you’d use only if you had no other choice.
The Chiplotle doc provides a wiring diagram for a suitable 9-to-25 pin cable, so I printed one and doodled on it while pondering the Great Cable Stash:
The color codes over on the left of the top diagram match a prebuilt cable I hoped to repurpose, but it had only five conductors, none of which were DSR or CTS. Pfui!
So I used a hank of gorgeous flexy 9-conductor cable (which came with premolded DE-9 ends of the wrong gender, now amputated into pigtails and back in the GCS), which supported the connections redrawn on the bottom in proper numeric order, used the obvious color sequence (Bn R O Y G Bl V W K), then soldered suitable connectors on each end:
And it worked the first time…
The Smell of Molten Projects in the Morning 
That wasn’t the saddest. The HP minicomputers in the late ’70s used ENQ/ACK for software handshaking. My first attempt to use a dialup link to work came to an abrupt halt, since the only terminals I could afford used XON/XOFF. So much for bleeding edge telecommuting. (Yet another example of HP doing things 45 degrees off the norm for the era. See PC, HP Vectra, “The least compatible of the IBM compatible computers.” quoth the salesman as I returned software that wouldn’t install.)
Turns out the 7475A powers up in “dummy” ␅/␆ handshake mode: if you send an ␅, it’ll cheerfully return an ␆ even with no buffer space available. You must manually set up the handshaking mode before it’ll actually work the way it should… seems HP wasn’t quite compatible with themselves.
Note to Self: those are Unicode “control pictures” U+2405 and U+2406, respectively:
http://www.fileformat.info/info/unicode/block/control_pictures/images.htm