python: fix signal-type label when overriding the plotted channel range

apply_conf_defaults() picked the plotted signal-type label as
signals[0].signal whenever --first-channel/--channels was passed
explicitly to override the conf-derived channel range. signals[0] is
just the first enabled signal in SIGNAL_ORDER for the given conf, so
it is only correct when the manually requested range happens to start
at channel 0 of that same signal; for any other range it mislabels
every plot with the wrong signal type, even though the correct .dat
files are read (since --file-prefix is typically also given
explicitly in that case).

For example, running
  --conf EVK1029_Galileo_E1_E5a.conf --file-prefix trk_GalileoE5a_ch \
  --first-channel 10 --channels 10
correctly reads the Galileo E5a tracking dumps but titled every plot
"Galileo E1B", the conf's first enabled signal.

Resolve each requested channel's signal type from the conf's own
per-signal channel ranges (SignalConfig.channels) instead, falling
back to the previous behavior only when no conf is available at all.

Signed-off-by: joebre <joel.brenner@saphyrion.ch>

utils/python/dll_pll_veml_plot_sample.py

@@ -184,13 +184,28 @@ def apply_conf_defaults(args):
     if args.file_prefix is None:
         args.file_prefix = DEFAULT_FILE_PREFIX
 
-    manual_signal = args.signal_type or (
+    manual_signal_fallback = args.signal_type or (
         signals[0].signal if signals else DEFAULT_SIGNAL_TYPE
     )
+
+    def signal_for_channel(channel):
+        # An explicit --first-channel/--channels range overrides the conf's
+        # auto-derived range, but the channel-to-signal mapping should still
+        # come from the conf when possible: signals[0] (first enabled signal
+        # in SIGNAL_ORDER) is not necessarily the signal actually occupying
+        # this channel index once the range has been overridden manually.
+        if args.signal_type is not None:
+            return args.signal_type
+        if conf is not None:
+            for signal in conf.signals:
+                if channel in signal.channels:
+                    return signal.signal
+        return manual_signal_fallback
+
     args.channel_plan = [
         ChannelDump(
             channel=channel,
-            signal=manual_signal,
+            signal=signal_for_channel(channel),
             file_prefix=args.file_prefix,
         )
         for channel in range(args.first_channel, args.first_channel + args.channels)