After posting about UI woes (and subsequently withdrawing that post), I started looking into Purr Data.
It appears to be quite difficult to install in Ubuntu 18.04.
At https://github.com/agraef/purr-data/releases, there is no .deb for Ubuntu 18 (only for 14 and 16). pd-l2ork-2.9.0-20190416-rev.2b3f27c-x86_64.deb fails to install:
dpkg: dependency problems prevent configuration of pd-l2ork: pd-l2ork depends on libgsl2; however: Package libgsl2 is not installed.
Ubuntu 18 does not have libgsl2. It has libgsl23 -- which is installed on my system, but does not satisfy the dependency.
https://launchpad.net/~dr-graef/+archive/ubuntu/pd-l2ork.bionic is supposedly a PPA for Ubuntu 18.04. After adding it:
$ sudo apt-get install purr-data The following packages have unmet dependencies: pd-l2ork : Depends: libgsl2 but it is not installable Recommends: ladspa-foo-plugins but it is not going to be installed Recommends: slv2-jack but it is not installable purr-data : Depends: libavifile-0.7c2 but it is not going to be installed Depends: libmpeg3-2 (>= 1.8.dfsg) but it is not going to be installed Recommends: ladspa-foo-plugins but it is not going to be installed E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).
I haven't looked into the "not going to be installed" packages, but I already know what "not installable" means -- at least, libgsl2 doesn't exist for my Ubuntu version -- https://packages.ubuntu.com/search?keywords=libgsl2 shows "Package libgsl2 -- xenial (16.04LTS)" only.
Neither agraef nor jonwwilkes' Purr Data github repositories allow issue reports (only pull requests). So I can't ask the distributors.
I guess I could try to build it myself...? But, if the currently available release packages are not really compatible with with current Ubuntu LTS, that's not a very good situation.