-- H,C HSQC
-- draws two arrows; 99% of HSQC peaks will fall between the two arrows
-- it is known that a relation exists betwee the coordinates of a cross peak:
-- H1_ppm = 0.053 13C_ppm + 0.852
-- the error can be as high as 2 or as low as -1.5 (tolerance)

local T = {}		-- tolerance
T[1] = 2			-- max positive error
T[2] = -1.5			-- max negative error (with minus sign)

local C13 = getf 'x'		-- coordinates of the spectrum, in ppm units
local H1 = getf 'y'
local transposed = false

if (C13.atom ~= 'C' and H1.atom ~= 'C') or (C13.atom ~= 'H' and H1.atom ~= 'H') then
	print "this script requires an hetero-correlated H-C experiment"
	return
end
if C13.atom == 'H' and H1.atom == 'C' then
	local swap = H1
	H1 = C13
	C13 = swap
	transposed = true
end	

local Xmin = C13.start					-- limits of the spectrum
local Xmax = C13.start + C13.width		
local Ymin = H1.start					
local Ymax = H1.start + H1.width			

for i = 1,2 do							-- for each arrow
	local fromX, fromY, toX, toY		-- coordinates of the arrow
	fromX = Xmin
	fromY = 0.053 * Xmin + 0.852 + T[i]
	if fromY < Ymin then
		fromY = Ymin
		fromX = (Ymin - 0.852 - T[i]) / 0.053
	end
	toX = Xmax
	toY = 0.053 * Xmax + 0.852 + T[i]
	if toY > Ymax then
		toY = Ymax
		toX = (Ymax - 0.852 - T[i]) / 0.053
	end
	if transposed then arrow( fromY, fromX, toY, toX, 0x380 )
	else arrow( fromX, fromY, toX, toY, 0x380 )	end -- hollow arrow with two short points
end

amp()