Define X axis output increments.

I would like to be able to define how many spaces between each value displayed on the X axis. For example if I have the post date as the value for the X axis and I only want every 4th date to be displayed I would like to be able to set that. So say there are 4 posts in a month and I would only like to show a value on the X axis for each month I would be able to set it to 4 meaning every 4th post. So options would be 1 for every post (or value), 2 for every other one, 3 for every third value, etc.

Again, I'm looking for a quote on this one so if anybody can do it for a price please let me know.

Thanks.