xyseries vs chart over by

Are there any differences between the following queries other than my observation (below):

1. xyseries Foo Bar Value
2. chart latest(Value) over Foo by Bar

I have noticed that chart last(Value) does not return (anymore?) non numeric results unless limit=0 is set? Which is weird and was a lucky find, but now I have an option, and I'm not sure how xyseries works over a timerange. For example I know latest(Value) pulls the most recent value for each key, even if they are not time aligned, so if I pick a large time scope (like 24 hours) I can interpolate into one row. xyseries does not seem to work exactly like this, but then again based on its behavior I cant figure out what its doing at all!

Any help is appreciated,