Some background
Pen plotters were the only choice for quality graphic output from computers from the 1950s to 1980s. Used by engineers, architects and businesses to produce drawings and presentations they were superseded by the emergence of affordable printing technologies – dot matrix, inkjet and laser. Plotters were also used from day one for creative purposes. If you want to learn more about that check out piratefsh's post.
And now, with the interest of digital natives, and near digital natives, they are experiencing a resurgence in interest and contemporary manufacturers such as Evil Mad Scientist have built accessible modern versions like the Axidraw SE/A3 plotter I use to create my drawings. Checkout #plottertwitter if you want to see a diverse range of people creating some awesome work.
The attraction of being able to output digital work, both generative and 'manually' created, using traditional analog tools and media is a nice loop back to the 'real world'.
My process
This varies a lot but at present I'm using photographs I've taken, developing them in Lightroom/Photoshop, processing them with a Python based tool called vpype, and its plugins like flow_imager, to create vector line artwork.
So it's a bit of a hybrid – not purely generative like a lot of plotter artwork but not manually created.
I didn't write the software myself (far too much tricky maths in the low level stuff for my abilities) but as it's open source have chatted with the developer(s) and other members of the community to make improvements, fix bugs and suggest new features.
The lines are created by setting variables (a lot of variables) which effect their length, direction, density, curves etc. etc. I don't know what the result will be before the program is run, and that can take 10-15 minutes for a complex A3 size plot, so I often set up a batch to run overnight.
In the morning I can peruse a folder of possibilities and select a few to play around with. By the time I've explored those I'll have a favourite which stands out, usually one which works really well with the subject of the source image. Often it's one I never would have arrived at manually changing variables which is a lovely part of the process.
That might then go through Illustrator/Inkscape for some final tweaks before it's sent to be drawn by the plotter. I send it to a small Raspberry Pi computer running a tool called saxi to drive the plotter without a 'big' computer attached. It's tools like this, created and shared by enthusiastic users, which make this community such a pleasure to be a part of.
Here's a timelapse of a plot running
Then there's a whole new layer to explore. More traditional creative choices: type of pen (/pencil/brush), paper stock, ink. As well as technical ones: speed of the plotter, time the pen takes to lower/lift etc.
It feels like a hybrid of drawing and printmaking. The machine draws faster and more accurately than a human (for better or worse) but it still takes the same time to produce each plotted drawing as the tool has to travel the same distance. There's no relatively rapid editioning of a printing press or screenprinting setup. But it's certainly not 'just press print'.
That's leading me to using the plotter to produce etched plates for intaglio printing – another rabbit hole I'm just starting down. Expect to see some etchings here at some stage :)