sed1330.c
Graphic LCD controllers have a bitmap memory that you can use to draw text or graphics. While you can address any dot on the screen randomly, there is a slight problem when you want to change 1 dot only. This is because eight pixels are represented by 1 byte. So what you normally do is that you read a byte, change the required bit and write back. That way no other pixel (text or graphic) is disturbed.
I have used T6963 based LCDs but I guess others are similar in nature. Some application notes including C code can be found via google that describes the operation in detail. You can port that code to your requirement.
One difference that i have found between T6963 and other controllers is that T6963 arranges 8 pixels horizontally in a byte while other do it vertically.