usrp/sps/: removed "Offset" from header, added 0 dB line to peak power overview

- row: the "Offset" header looked very crowded; removed it
- vizp: draw the x axis (0 dB) as solid black

usrp/sps/row

@@ -24,7 +24,7 @@ for prefix in "$@"; do
 	fi
 	last=$prefix
 
-	echo "<TR$color><TH>$prefix<TD align=\"center\">Offset"
+	echo "<TR$color><TH>$prefix<TD>"
 	f=2405
 	while [ $f -le 2480 ]; do
 		echo "<TD align=\"center\">$f MHz"

usrp/sps/vizp

@@ -45,6 +45,7 @@ set yrange [-20:20]
 set xlabel "Frequency of test wave (MHz)"
 set ylabel "Peak power (dB)"
 
+set xzeroaxis lt -1
 set mytics 2
 set grid