How change "Previous Data" to something else in Diff Charts?

[Steve Ricketts]

I can't seem to find out how to change the legend in a Diff Chart from "Previous Data" to a custom value.  I'm adding the labels like:

[code]

data.addColumn( 'string', oInfo.id );

data.addColumn( 'number', oInfo.label );

data2.addColumn( 'string', oInfo2.id ); 

data2.addColumn( 'number', oInfo2.label );

var diffData = chart.computeDiff(data2,data);

chart.draw(diffData, options);

[/code]

The first label comes out correctly but the second one always says "Previous Data".  How do I replace the label "Previous Data" with my custom label?