[Scroll pHat HD] Is it possible to make a numbers only custom font?

I want to make a custom 3x11 font containing just numbers and a colon (for a clock). Is this possible, or do I need to make an entire character set? I’ve had a go with this png:

3x11-font

and this setup in mkfont.py:

FONT_FILE = "3x11-font.png"

SHEET_WIDTH = 11
SHEET_HEIGHT = 1

FONT_WIDTH = 3
FONT_HEIGHT = 11

CHAR_SPACING_X = 0
CHAR_SPACING_Y = 0
MARGIN_X = 0
MARGIN_Y = 0

CHAR_START = 0x30
CHAR_END = 0x3a

But all I get from python mkfont.py > 3x11-font.py is a zero-byte output file and this error:

Traceback (most recent call last):
  File "mkfont.py", line 110, in <module>
    font = kern_font(load_font(FONT_FILE))
  File "mkfont.py", line 73, in load_font
    font[char] = get_char_data(font_image, px, py)
  File "mkfont.py", line 57, in get_char_data
    palette_index = font_image.getpixel((o_x + x, o_y + y))
  File "/usr/lib/python2.7/dist-packages/PIL/Image.py", line 1333, in getpixel
    return self.im.getpixel(xy)
IndexError: image index out of range

This is on Raspbian Buster Lite. Python Pillow is installed via pip. The mkfont.py and 3x11-font.png files are in the ~/Pimoroni/scrollphathd/tools folder. Any help appreciated, thanks.

Okay, so it turns out I’m a massive idiot. Somehow my brain started believing the depth of the display was 11 pixels when it was only seven pixels. I am attempting to remove my idiocy from the equation, which could very well simplify matters.