Simply take the patch mentioned in the redmine bug 4423 for offset, and apply the same idea to frequency:
--- rrd.inc.old 2016-07-01 16:29:15.917333000 -0400
+++ rrd.inc 2016-07-01 16:23:52.048739000 -0400
@@ -912,7 +912,7 @@
$rrdcreate .= "DS:sjit:GAUGE:$ntpdvalid:0:1000 ";
$rrdcreate .= "DS:cjit:GAUGE:$ntpdvalid:0:1000 ";
$rrdcreate .= "DS:wander:GAUGE:$ntpdvalid:0:1000 ";
- $rrdcreate .= "DS:freq:GAUGE:$ntpdvalid:0:1000 ";
+ $rrdcreate .= "DS:freq:GAUGE:$ntpdvalid:-1000:1000 ";
$rrdcreate .= "DS:disp:GAUGE:$ntpdvalid:0:1000 ";
$rrdcreate .= "RRA:MIN:0.5:1:1200 ";
$rrdcreate .= "RRA:MIN:0.5:5:720 ";
Works for me; but as before you have to rm /var/db/rrd/ntpd.rrd and then recreate it (In services/ntp/settings, uncheck rrd logging, save, then check rrd logging, save).
But, with typical negative frequency values, this will screw up the graph scaling terribly, so this is probably not the best solution with the current graphing back-end.