Flowing grouped data

When calling .flow() on grouped data when values are also being pushed off the chart causes an error. Here's an example: http://jsfiddle.net/zavy2cwu/

It also causes some interesting looking results when using a bar chart: http://jsfiddle.net/3jhLau3u/

If flow doesn't push any data off the chart (length: 0), everything seems to be fine: http://jsfiddle.net/aL0sjLkz/

If the data isn't grouped, there aren't any errors, but with bar chart the animation compresses the bars and they expand out again: http://jsfiddle.net/fu41zeg8/