I agree with Diana--and that's what I usually do when I see a chart like that, I just work in some 1/4 stitches and it usually smooths it out to my liking
Work in Progress
Dimensions Passion Flower Angel
Tempting Tangles Welcome SAL
Various small designs
The only other way I can think of in addition to those already listed is to work it the same size (dimensions wise, rather than stitch count) on a much higher count fabric. That way the circle has to go over many more lines of fabric to be complete and whilst it still has jaggies, it's much less noticeable than the same size circle on a low count fabric.
WIP
Rose Window
SQ Sunday afternoon on the Island of La Grande Jatte - pgs 1-3 complete
HAED Curl up with a Good Book
Around the World in 80 Stitches - pts 1-12 complete
Mabel's SOTW - Jan-June complete
HAED BB Sal
I've also found that what looks really jagged up close, your eye and mind fill in when you see it from a normal distance. It also helps if you weren't the one who stitched it and knows where all the jagged edges are.
I'm working on a chart I made that is just font, although a slightly decorative one. I'm adjusting it on the fly and keep seeing all these little details that seem like huge issues to me. Then I show it my husband and he says "Yep, looks like an O".
I do agree with others though, the fractional stitches can really make a difference.
**Signature Under Construction**
(will update soon with current WIPs and other info )
The 'jaggies' is why I didn't care for cross stitch much until I saw the more detailed patterns that are available these days. You could take a cue from them and use a color that's inbetween the moon and the sky to help form a smoother transition between the two. That is what the HAED pattern I'm working on does for the black outlines on the pattern. And the end result is flawless!
I have the same problem, specially when I am trying to design my own patterns. I have one large piece that I am working on and calls for loads of half and quarter stitches as the pattern has polka dots but I cant seem to make it look neat enough. I should either learn to half stitch properly or lean to live with the "jaggies".