I have the same issue, but know there is little I can do.
The problem is with Outlook 2007. I can see the graphs in a variety other Web Browsers and Mail applications that support HTML.
It is because Outlook uses Word to do it's HTML rendering. Word's HTML engine doesn't support much beyond some basic HTML abilities, so things like the Availability Graphs from ipMonitor do not display.
Maybe Office 2010 will fix this? Maybe not... all I know is the graphs turn up ok on my iPhone :)