As I've been editing code to not show warnings in the PHP error log, I have made lots of edits to draw_grid_png.php. I'll have to take a look at it and see what's causing this.
It does not appear to be caused by any recent edits to draw_grid_png.php. I temporarily reverted to an earlier version, and the problem remained.
It does not appear to be caused by any recent edits to draw_grid_png.php. I temporarily reverted to an earlier version, and the problem remained.