Something is wrong with the line algoritm.

--[[
	[Library] Graphics
	@version 1.0, 2015-02-23
	@author TheOddByte
--]]



local Graphics = {}




Graphics.setColor = function( col )
	assert( type( col ) == "number" or type( col ) == "string", "string/number expected, got " .. type( col ), 2 )
	term.setTextColor( type( col ) == "number" and col or colors[col] )
end



Graphics.setBackgroundColor = function( col )
	assert( type( col ) == "number" or type( col ) == "string", "string/number expected, got " .. type( col ), 2 )
	term.setBackgroundColor( type( col ) == "number" and col or colors[col] )
end



Graphics.writeAt = function( x, y, text )
	term.setCursorPos( x, y )
	term.write( text )
end



Graphics.line = function( x1, y1, x2, y2, col )
	local dx, dy = x2-x1, y2-y1

	if col then
		Graphics.setBackgroundColor( col )
	end

	Graphics.writeAt( x1, y1, " " )
	if (dx ~= 0) then
		local m = dy / dx;
		local b = y1 - m*x1;
		dx = x2 > x1 and 1 or -1
		while x1 ~= x2 do
			x1 = x1 + dx
			y1 = math.floor( m*x1 + b + 0.5);
			Graphics.writeAt( x1, y1, " " )
		end
	end
end



Graphics.rectangle = function( mode, x, y, width, height, col )
	assert( mode == "fill" or mode == "line", "invalid mode", 2 )
	Graphics.setBackgroundColor( col )
	local line = string.rep( " ", width )
	if mode == "fill" then
		for i = 1, height do
			Graphics.writeAt( x, (y-1)+i, line )
		end
	else
		for i = 1, height do
			if i == 1 or i == height then
				Graphics.writeAt( x, (y-1)+i, line )
			else
				Graphics.writeAt( x, (y-1)+i, " " )
				Graphics.writeAt( x + width - 1, (y-1)+i, " " )
			end
		end
	end
end



return Graphics

local Graphics = dofile( "Graphics" )

local line = {
	x1 = 1,
	y1 = 3,
	x2 = 51;
	y2 = 19;
}


local mode = "graphics"
while true do
	Graphics.setBackgroundColor( "black" )
	term.clear()
	Graphics.writeAt( 1, 1, mode == "graphics" and "Graphics API" or "Paintutils" )
	if mode == "graphics" then
		Graphics.line( line.x1, line.y1, line.x2, line.y2, "white" )
	else
		paintutils.drawLine( line.x1, line.y1, line.x2, line.y2, colors.red )
	end
	local e = { os.pullEvent() }
	if e[1] == "mouse_click" or e[1] == "mouse_drag" then
		if e[2] == 1 then
			line.x1, line.y1 = e[3], e[4]

		elseif e[2] == 2 then
			line.x2, line.y2 = e[3], e[4]
		end

	elseif e[1] == "key" then
		if e[2] == 28 then
			mode = mode == "graphics" and "paintutils" or "graphics"
		end

	elseif e[1] == "char" then
		if e[2] == "r" then
			local w, h = term.getSize()
			line.x1, line.y1 = math.random( 1, w ), math.random( 1, h )
			line.x2, line.y2 = math.random( 1, w ), math.random( 1, h )
		end
	end
end