In my Customer Portal, the pie chart shows incomplete background (rectangle) on hover tips over the wedges. I can make tips="false" to not show the hover tips, but I need the hover tips. How do I make the rectangular background span the whole content of the tips?

<apex:pageBlockSection columns="1">
<apex:chart data="{!Summaries}" height="200" width="200">
<apex:pieSeries labelField="careerLatticeLevel"
dataField="recordCount" donut="50" showInLegend="true" />
</apex:chart>
</apex:pageBlockSection>