1. Go to Properties (of the chart) > Presentation > Text in Chart (bottom left). Add your text, select the chart and press ctrl+shift to display red boxes around objects within the chart: now drag and drop the new text object the where you need.
2. Create a new Text object and firstly set the transparency to 100% and then under the Layout tab of the Properties set the layer to be 'Top'. Position accordingly over your chart.
Hope that helps,
Matt - Visual Analytics Ltd