herctrap:
this code
GLCD.DrawLine(31,31,31,0);
creates a vertical line not a horizontal
from the reference doc:
void DrawLine (uint8_t x1, uint8_t y1, uint8_t x2, uint8_t y2, uint8_t color=BLACK)
As I read it, the second pair of values for DrawLine are coordinates not sizes, so I would expect the given code to draw a vertical line.