labelRotation in charts - but the labels remain unchanged

Trying to get x-axis labels in vertical orientation.  The attribute labelRotation is 90.0, but the chart displays the labels horizontally.

           <plot foregroundAlpha="0.5" labelRotation="90.0">
                        <seriesColor seriesOrder="0" color="#33CC00"/>
                        <seriesColor seriesOrder="1" color="#FF3333"/>
                        <seriesColor seriesOrder="2" color="#3333FF"/>
             </plot>

What do i wrong?

Thank you for help.

Michael Fischer