Bench: --plug hp — the HP t740 (demo-hp) is on a metered plug too

plugmeter.py and plugswitch.py take --plug n100|hp (plug.json / plug-hp.json, outside the repo).
The HP plug read 23-33 W while the box started its three guests.

scripts/bench/plugswitch.py

@@ -4,6 +4,7 @@
   ~/.venvs/plugmeter/bin/python scripts/bench/plugswitch.py status
   ~/.venvs/plugmeter/bin/python scripts/bench/plugswitch.py off
   ~/.venvs/plugmeter/bin/python scripts/bench/plugswitch.py on
+  ~/.venvs/plugmeter/bin/python scripts/bench/plugswitch.py status --plug hp   # the HP t740's plug
 
 Reads ~/.config/felhom-bench/plug.json (the same file plugmeter.py reads). `off` also needs, in that file:
   "standby_w":  the plug's measured reading with the machine powered off (the A2 figure);
@@ -18,14 +19,14 @@ The key and the device id are never printed. Pinned by scripts/bench/test_plugsw
 """
 import json, os, subprocess, sys, time
 
-CONF = os.path.expanduser('~/.config/felhom-bench/plug.json')
+PLUG_FILES = {'n100': 'plug.json', 'hp': 'plug-hp.json'}  # --plug, as plugmeter.py
 WINDOW_S = 20
 STEP_S = 2
 MARGIN_W = 1.0
 
 
-def load():
-    return json.load(open(CONF))
+def load(plug='n100'):
+    return json.load(open(os.path.expanduser('~/.config/felhom-bench/' + PLUG_FILES[plug])))
 
 
 def device(c):
@@ -71,11 +72,14 @@ def off_allowed(samples, standby_w, window_s=WINDOW_S):
 
 
 def main(argv):
+    plug = 'n100'
+    if len(argv) == 4 and argv[2] == '--plug' and argv[3] in PLUG_FILES:
+        plug, argv = argv[3], argv[:2]
     if len(argv) != 2 or argv[1] not in ('on', 'off', 'status'):
-        print('usage: plugswitch.py on|off|status', file=sys.stderr)
+        print('usage: plugswitch.py on|off|status [--plug n100|hp]', file=sys.stderr)
         return 64
     cmd = argv[1]
-    c = load()
+    c = load(plug)
     dev = device(c)
     if cmd == 'status':
         w, on = read_watts(dev, c)