If someone looking for the solution.

You need to wait the chart rendered its elements (like a bar).

I added a Thread.Sleep(1000) after the forced rendered and before to save the visual as png. And it work !

 chart.Measure(new Size(Double.PositiveInfinity, Double.PositiveInfinity));
 chart.Arrange(new Rect(userControl.DesiredSize));
 Thread.Sleep(900);
 // and save as png

Many thanks to Gerry in comments !