Can't load x and y data separately

Hello.

I have a simple XY chart (with series `data1` and `x`) where i'd like to `.load` new data one series at a time (first `x`, then `data1`), but c3 seem to ignore it. However if i load both series at them same time - it works. Is it correct behavior?

See the [fiddle](http://jsfiddle.net/nidu/c52Lsgzh/).