Get the dataprovider of another Line in lineChart?

In some cases you have 2 different XML files to show 2 different lines in a lineChart. For example you want to compare a companies incomming with another month and each month is in a different XML. In a dataTipRenderer I show the styled data but I can only access data from 1 line. My goal is to create something like this in my dataTip:

April 2009
$29,000
May 2009
$30,000
+3%

Is there a way to get a dataProvider from the other line that is in the lineChart? Or is there a way to work around this?