Where do the temporary files live?

By default, the OS temp directory.

You may specify the location of temporary files through configuration. Either by having a suitably located "application.conf", or by passing in the environment variable DEDAV_OUT_PATH.

e.g.

sbt -DDEDAV_OUT_PATH=/Users/simon/Pictures   

Assuming you have a chart, produced with one of the desktop targets (browser, tempFile, png, svg, pdf)

import viz.vega.plots.{BarChart}
import viz.PlotTargets.tempFileSpec
val chart = BarChart()
// chart: BarChart = BarChart(mods = List())
chart.tmpPath
// res0: Option[Path] = Some(value = /tmp/plot-14813590224738042547.txt)

It's location is in the tmpPath field of your chart. You could take advantage of the excellent os library and knowledge of the path of the file, to subsequently move it around. For example if you wish to automate producing a presentation via a script.