I opened a second ticket on this and finally got a response:
From: "Weather Underground, Inc" <support@wunderground.com>
Sent: Thursday, April 16, 2009 7:51:19 AM
Subject: [#GLX-623058]: Wind direction graph
This is by design, and probably won't be fixed.
Thank you for your support!
Weather Underground, Inc
http://www.wunderground.com/Ticket Details
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Ticket ID: GLX-623058
Category: Personal Weather Stations