Unoptimized conversion works, let's see if it breaks the calculator.

demo.lua

@@ -1,16 +1,13 @@
 local bitmap = require'bitmap' --used to read its content
 
 function int_exa(nb)
-  output = "0123456789abcdef"
-  first_digit = 0
-  while nb > 15 do
-    nb = nb - 16
-    first_digit = first_digit + 1
-  end
-  return output:sub(first_digit + 1, 1)..output:sub(nb + 1, 1)
+  if nb < 16 then add = "0"
+  else add = "" end
+  return [[\x]]..add..string.format("%x", nb)
 end
 
-io.write("from casioplot import set_pixel, show_screen as S, R\n")
+io.write([[from casioplot import set_pixel, show_screen as S, R
+data=b"]])
 
 do --open image
   local cairo = require'cairo' --used to open the file
@@ -23,7 +20,6 @@ local getpixel, setpixel = bitmap.pixel_interface(image_bitmap)
 pallet = {} --will store colors
 pallet_last = 0
 
-
 for j = 1, image_infos.height, 1 do
   for i = 1, image_infos.width, 1 do
     r, g, b = getpixel(i, j)
@@ -35,4 +31,6 @@ for j = 1, image_infos.height, 1 do
   end
 end
 
-io.write("\nR()")
+io.write([["
+R()
+]])