Turns out it's probably Chromium crashing, as determined from running the command line (obtained from ps(1) after clicking to generate an image) and running it by hand:
-bash-4.2$ /var/lib/grafana/plugins/grafana-image-renderer/chrome-linux/chrome --disable-background-networking --enable-features=NetworkService,NetworkServiceInProcess --disable-background-timer-throttling --disable-backgrounding-occluded-windows --disable-breakpad --disable-client-side-phishing-detection --disable-component-extensions-with-background-pages --disable-default-apps --disable-dev-shm-usage --disable-extensions --disable-features=TranslateUI,BlinkGenPropertyTrees --disable-hang-monitor --disable-ipc-flooding-protection --disable-popup-blocking --disable-prompt-on-repost --disable-renderer-backgrounding --disable-sync --force-color-profile=srgb --metrics-recording-only --no-first-run --enable-automation --password-store=basic --use-mock-keychain --headless --hide-scrollbars --mute-audio about:blank --no-sandbox --remote-debugging-port=0 --user-data-dir=/tmp/puppeteer_dev_profile-NisVM0
[1117/132856.231039:FATAL:thread_helpers.cc(44)] Check failed: 3UL <= task_stat.st_nlink (3 vs. 2)
#0 0x7fffea5bce29 <unknown>
#1 0x7fffea520ee3 <unknown>
#2 0x7fffea5356b3 <unknown>
#3 0x7fffebc9b0a9 <unknown>
#4 0x7fffebc9b112 <unknown>
#5 0x7fffebd18a4c <unknown>
#6 0x7fffea15ad3e <unknown>
#7 0x7fffea15c224 <unknown>
#8 0x7fffea1a8acd <unknown>
#9 0x7fffea15a541 <unknown>
#10 0x7fffea1a79dd <unknown>
#11 0x7fffea1a78bf <unknown>
#12 0x7fffea1a61c5 <unknown>
#13 0x7fffe7e6c218 <unknown>
#14 0x7fffdae22555 <unknown>
#15 0x7fffe7e6c0aa <unknown>
Received signal 6
#0 0x7fffea5bce29 <unknown>
#1 0x7fffea520ee3 <unknown>
#2 0x7fffea5bc9b1 <unknown>
#3 0x7fffe480f630 <unknown>
#4 0x7fffdae36387 <unknown>
#5 0x7fffdae37a78 <unknown>
#6 0x7fffea5bb815 <unknown>
#7 0x7fffea535af9 <unknown>
#8 0x7fffebc9b0a9 <unknown>
#9 0x7fffebc9b112 <unknown>
#10 0x7fffebd18a4c <unknown>
#11 0x7fffea15ad3e <unknown>
#12 0x7fffea15c224 <unknown>
#13 0x7fffea1a8acd <unknown>
#14 0x7fffea15a541 <unknown>
#15 0x7fffea1a79dd <unknown>
#16 0x7fffea1a78bf <unknown>
#17 0x7fffea1a61c5 <unknown>
#18 0x7fffe7e6c218 <unknown>
#19 0x7fffdae22555 <unknown>
#20 0x7fffe7e6c0aa <unknown>
r8: 0000000000000000 r9: 0000000000000000 r10: 0000000000000008 r11: 0000000000000246
r12: 00007fffffefddd8 r13: 00007fffffefcd28 r14: 00007fffffefdde0 r15: 00007fffffefcd40
di: 000000000000f7f0 si: 000000000000f7f0 bp: 00007fffffefccd0 bx: 00007fffffefcd28
dx: 0000000000000006 ax: 0000000000000000 cx: 0000000000000008 sp: 00007fffffefcb98
ip: 00007fffdae36387 efl: 0000000000000206 cgf: 004b000000000053 erf: fffffffffb8005fc
trp: 000000000000000e msk: 0000000000000001 cr2: 0000000000000000
[end of stack trace]
Calling _exit(1). Core file will not be generated.
DevTools listening on ws://127.0.0.1:51823/devtools/browser/6bcc68ff-cad1-49c3-806b-42cfeb1f515f
[1117/132858.293996:FATAL:thread_helpers.cc(44)] Check failed: 3UL <= task_stat.st_nlink (3 vs. 2)
#0 0x7fffea5bce29 <unknown>
#1 0x7fffea520ee3 <unknown>
#2 0x7fffea5356b3 <unknown>
#3 0x7fffebc9b0a9 <unknown>
#4 0x7fffebc97fcd <unknown>
#5 0x7fffebc98728 <unknown>
#6 0x7fffee479114 <unknown>
#7 0x7fffebb4fc61 <unknown>
#8 0x7fffee4788f9 <unknown>
#9 0x7fffea15c224 <unknown>
#10 0x7fffea1a8acd <unknown>
#11 0x7fffea15a541 <unknown>
#12 0x7fffea1a79dd <unknown>
#13 0x7fffea1a78bf <unknown>
#14 0x7fffea1a61c5 <unknown>
#15 0x7fffe7e6c218 <unknown>
#16 0x7fffdae22555 <unknown>
#17 0x7fffe7e6c0aa <unknown>
Received signal 6
#0 0x7fffea5bce29 <unknown>
#1 0x7fffea520ee3 <unknown>
#2 0x7fffea5bc9b1 <unknown>
#3 0x7fffe480f630 <unknown>
#4 0x7fffdae36387 <unknown>
#5 0x7fffdae37a78 <unknown>
#6 0x7fffea5bb815 <unknown>
#7 0x7fffea535af9 <unknown>
#8 0x7fffebc9b0a9 <unknown>
#9 0x7fffebc97fcd <unknown>
#10 0x7fffebc98728 <unknown>
#11 0x7fffee479114 <unknown>
#12 0x7fffebb4fc61 <unknown>
#13 0x7fffee4788f9 <unknown>
#14 0x7fffea15c224 <unknown>
#15 0x7fffea1a8acd <unknown>
#16 0x7fffea15a541 <unknown>
#17 0x7fffea1a79dd <unknown>
#18 0x7fffea1a78bf <unknown>
#19 0x7fffea1a61c5 <unknown>
#20 0x7fffe7e6c218 <unknown>
#21 0x7fffdae22555 <unknown>
#22 0x7fffe7e6c0aa <unknown>
r8: 0000000000000000 r9: 0000000000000000 r10: 0000000000000008 r11: 00007fffdaf8dd10
r12: 00007fffffefd788 r13: 00007fffffefc6d8 r14: 00007fffffefd790 r15: 00007fffffefc6f0
di: 000000000000f7fc si: 000000000000f7fc bp: 00007fffffefc680 bx: 00007fffffefc6d8
dx: 0000000000000006 ax: 0000000000000000 cx: 0000000000000008 sp: 00007fffffefc548
ip: 00007fffdae36387 efl: 0000000000000206 cgf: 004b000000000053 erf: fffffffffb8005fc
trp: 000000000000000e msk: 0000000000000001 cr2: 0000000000000000
[end of stack trace]
Calling _exit(1). Core file will not be generated.
[1117/132900.492947:WARNING:gpu_process_host.cc(1204)] The GPU process has crashed 1 time(s)
[1117/132900.603763:FATAL:thread_helpers.cc(44)] Check failed: 3UL <= task_stat.st_nlink (3 vs. 2)
#0 0x7fffea5bce29 <unknown>
#1 0x7fffea520ee3 <unknown>
#2 0x7fffea5356b3 <unknown>
#3 0x7fffebc9b0a9 <unknown>
#4 0x7fffebc97fcd <unknown>
#5 0x7fffebc98728 <unknown>
#6 0x7fffee479114 <unknown>
#7 0x7fffebb4fc61 <unknown>
#8 0x7fffee4788f9 <unknown>
#9 0x7fffea15c224 <unknown>
#10 0x7fffea1a8acd <unknown>
#11 0x7fffea15a541 <unknown>
#12 0x7fffea1a79dd <unknown>
#13 0x7fffea1a78bf <unknown>
#14 0x7fffea1a61c5 <unknown>
#15 0x7fffe7e6c218 <unknown>
#16 0x7fffdae22555 <unknown>
#17 0x7fffe7e6c0aa <unknown>
Received signal 6
#0 0x7fffea5bce29 <unknown>
#1 0x7fffea520ee3 <unknown>
#2 0x7fffea5bc9b1 <unknown>
#3 0x7fffe480f630 <unknown>
#4 0x7fffdae36387 <unknown>
#5 0x7fffdae37a78 <unknown>
#6 0x7fffea5bb815 <unknown>
#7 0x7fffea535af9 <unknown>
#8 0x7fffebc9b0a9 <unknown>
#9 0x7fffebc97fcd <unknown>
#10 0x7fffebc98728 <unknown>
#11 0x7fffee479114 <unknown>
#12 0x7fffebb4fc61 <unknown>
#13 0x7fffee4788f9 <unknown>
#14 0x7fffea15c224 <unknown>
#15 0x7fffea1a8acd <unknown>
#16 0x7fffea15a541 <unknown>
#17 0x7fffea1a79dd <unknown>
#18 0x7fffea1a78bf <unknown>
#19 0x7fffea1a61c5 <unknown>
#20 0x7fffe7e6c218 <unknown>
#21 0x7fffdae22555 <unknown>
#22 0x7fffe7e6c0aa <unknown>
r8: 0000000000000000 r9: 0000000000000000 r10: 0000000000000008 r11: 00007fffdaf8dd10
r12: 00007fffffefd788 r13: 00007fffffefc6d8 r14: 00007fffffefd790 r15: 00007fffffefc6f0
di: 000000000000f800 si: 000000000000f800 bp: 00007fffffefc680 bx: 00007fffffefc6d8
dx: 0000000000000006 ax: 0000000000000000 cx: 0000000000000008 sp: 00007fffffefc548
ip: 00007fffdae36387 efl: 0000000000000206 cgf: 004b000000000053 erf: fffffffffb8005fc
trp: 000000000000000e msk: 0000000000000001 cr2: 0000000000000000
[end of stack trace]
Calling _exit(1). Core file will not be generated.
[1117/132902.780787:WARNING:gpu_process_host.cc(1204)] The GPU process has crashed 2 time(s)
^C
Maybe because we're running Grafana in a lx-brand zone? Anyway, giving up for now. Could try redeploying as BHYVE.