The coorindate system used by the openGLCD library is not a normal cartesian coordinate system.
The coordinate system used by the library assigns (0,0) to the upper left corner. Y values grow down instead of the normal up direction, but X values increment left to right as in normal cartesian coordinates.
Below is an example of the coordinate system for a 128x64 glcd. It also includes the geometry properties.
0,0 is the upper left edge of the display.
Properties
GLCD.Width is the width of the display in pixels
GLCD.Height is the height of the display in pixels
GLCD.Left is the left most pixel x coordinate (0 zero)
GLCD.Right is the right most pixel x coordinate (equals GLCD.Width-1)
GLCD.Top is the top most pixel y coordinate on the display (0 zero)
GLCD.Bottom is the bottom pixel y coordinate (equals GLCD.Height-1)
GLCD.CenterX is the horizontal center (equals GLCD.Width/2)
GLCD.CenterY is the vertical center (equals GLCD.Height/2)