Grid classes are not correct when grids are dynamically added w/ chart.xgrids

Hi there,

First off; nice project, this is awesome! I've gotten to the point where I'm noticing strange behavior when I am adding and removing x grids via chart.xgrids([{...}, {...}]). I think the only reason I'm noticing this behavior is that I'm using the class property and changing the colors of the grid lines. Without the color, I wouldn't have noticed this.

I've put together a fiddle to illustrate the behavior.

https://jsfiddle.net/a7hap5zm/1/

The expected results should be:
1. Blue line @ 1
2. Blue line @ 1, red line @ 3
3. Red line @ 3
4. Red line @ 3, blue line @ 1

Instead, what I get is:
1. Blue line @ 1
2. Blue line @ 1, red line @ 3
3. Blue line @ 3
4. Blue line @ 3, blue line @ 1

It seems like the class name attribute of the grid nodes aren't being changed to reflect the desired class name.