title: Oxyplot: another value in label
body: <p>is it possible to show another value in a label of a series or to compute the value for a label?
  Background: I need a columnseries with TimeSpan-Values. I can use the Axis.LabelFormatter to show Timespan values on the axis. But there is no LabelFormatter for the series. How can I show Timespan values in the labels?</p>