First some clarification of the working memory theory.
Miller never said that people can only keep seven things (+/- 2) in their short term memory. His paper was about working memory (an entirely different concept). There are various numbers (note the plural) representing the chunks of info we can work with that are affected by context and type of information. As Miller and numerous others have found, they are generally lower than 7, with some types of info being as low as 1.
I hate citing wikipedia for something like this, but theirs is the neatest layman's write-up I have seen: http://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_or_Minus_Two
With specific regards the bar chart - users will typically only compare two bars at a time (by compare I mean not just look for the blunt affect of higher and lower, but actually check the numbers represented). They will fix one bar as the standard, and then scan the other bars measuring each against the standard, one at a time. The entire graph can be scanned for the trend over time - erikrojo's highs and lows - but that's a different form of comparison.
In short - I'd recommend you use as many as you need to provide meaning to the numbers in your context. If it's too many on the x-axis (requiring a horizontal scroll), consider providing a means for the user to freeze a section of the graph or select the number to display per jonshariat's answer.