scripts: plot.py: Reorganized main -> main_ to isolate -k/--keep-open logic

This simplifies plot.py's -k/--keep-open logic into a self-contained
loop that just calls main_ on an update.

This is a compromise on getting rid of -k/--keep-open completely, since
we _could_ just rely on watch.py. But plot.py knowing which argument is
the file to watch is convenient.

The eventual plan is to adopt this small bit of copy-pastable-code in
the other ascii-art scripts (treemap.py, dbgbmap.py, etc).

scripts/plot.py

@@ -1070,7 +1070,9 @@ class Grid:
         return grid
 
 
-def main(csv_paths, *,
+# TODO adopt main_ in all result scripts? (vs report)
+# TODO adopt this keep_open pattern in all ascii-art scripts?
+def main_(f, csv_paths, *,
         by=None,
         x=None,
         y=None,
@@ -1104,11 +1106,14 @@ def main(csv_paths, *,
         legend_below=False,
         subplot={},
         subplots=[],
-        head=False,
-        cat=False,
         keep_open=False,
-        sleep=None,
         **args):
+    # give f an writeln function
+    def writeln(s=''):
+        f.write(s)
+        f.write('\n')
+    f.writeln = writeln
+
     # figure out what color should be
     if color == 'auto':
         color = sys.stdout.isatty()
@@ -1255,11 +1260,7 @@ def main(csv_paths, *,
         )
 
 
-    def draw(f):
-        def writeln(s=''):
-            f.write(s)
-            f.write('\n')
-        f.writeln = writeln
+    ## our main drawing logic
 
     # first collect results from CSV files
     fields_, results = collect(csv_paths)
@@ -1553,7 +1554,7 @@ def main(csv_paths, *,
         s.yticklabels_ = subyticklabels
 
 
-        # now that everything's plotted, let's render things to the terminal
+    ## now that everything's plotted, let's render things to the terminal
 
     # figure out margin
     xmargin = (
@@ -1724,23 +1725,36 @@ def main(csv_paths, *,
                         for j in range(min(legend_cols, len(legend_)-i)))))
 
 
+def main(csv_paths, *,
+        keep_open=False,
+        head=False,
+        cat=False,
+        sleep=False,
+        **args):
+    # note main_ still wants keep-open for header padding
+
+    # keep-open?
     if keep_open:
         try:
             while True:
                 # register inotify before running the command, this avoids
                 # modification race conditions
-                if keep_open and Inotify:
+                if Inotify:
                     inotify = Inotify(csv_paths)
 
                 if cat:
-                    draw(sys.stdout)
+                    main_(sys.stdout, csv_paths,
+                            keep_open=False,
+                            **args)
                 else:
                     ring = RingIO(head=head)
-                    draw(ring)
+                    main_(ring, csv_paths,
+                            keep_open=True,
+                            **args)
                     ring.draw()
 
                 # try to inotifywait
-                if keep_open and Inotify:
+                if Inotify:
                     ptime = time.time()
                     inotify.read()
                     inotify.close()
@@ -1753,8 +1767,12 @@ def main(csv_paths, *,
 
         if not cat:
             sys.stdout.write('\n')
+
+    # single-pass?
     else:
-        draw(sys.stdout)
+        main_(sys.stdout, csv_paths,
+                keep_open=False,
+                **args)
 
 
 if __name__ == "__main__":
@@ -1984,6 +2002,10 @@ if __name__ == "__main__":
             '--subplot',
             type=AppendSubplot.parse,
             help="Add subplot-specific arguments to the main plot.")
+    parser.add_argument(
+            '-k', '--keep-open',
+            action='store_true',
+            help="Continue to open and redraw the CSV files in a loop.")
     parser.add_argument(
             '-^', '--head',
             action='store_true',
@@ -1992,10 +2014,6 @@ if __name__ == "__main__":
             '-z', '--cat',
             action='store_true',
             help="Pipe directly to stdout.")
-    parser.add_argument(
-            '-k', '--keep-open',
-            action='store_true',
-            help="Continue to open and redraw the CSV files in a loop.")
     parser.add_argument(
             '-s', '--sleep',
             type=float,