To generate graphs the Mega1280 is not equipped with enough RAM. You'll get such things a lot cheaper if you use one of the routers capable of running OpenWRT, there you have all the graphing utilities you need for such stuff and the WiFi for free. If you buy one with an USB port you can connect a simple Arduino (like the UNO) to do the low level stuff.
Also is it even posible for the RN-XV/WiFly to upload data from an SD Card?
Yes, this is possible but it won't be very fast.
If you wanna use your RN-XV, use it only for the data transfer to a web server and do all the graphical stuff there.