When it comes to DateTime conversions like this, RadChart doesn't allow for anything but a solid range (aka, no gaps in the dates like you are describing) so from 1/1/2009 to 7/7/2009 every day or month (depending on what you do with your DateTime) would have to be included on the x-axis ( I learned this the hard way :[ ).
My suggestions would be to either style
these days differently, or manually create your x-axis with integer values and then "mask"
these by setting the text of each integer to the corresponding date. This way you can exclude whatever dates you would want to exclude.
I hope my above suggestions make sense.