tick_spec = function(lo, hi)
	-- given an interval of numbers, return a nice set of ticks to draw for the interval
	-- the ticks will try to be at multiples of 10, or some multiple of a power of 10
	-- we don't want to overwhelm the viewer, so we'll aim for between 2 and 4 ticks
	-- the ticks will be specified as a low and high value, and a delta between consecutive ticks
	local anchorlo, scale = approximate(lo, order_of_magnitude(hi-lo))
	local anchorhi = approximate_up(hi, order_of_magnitude(hi-lo))
--	print('---', lo, hi, order_of_magnitude(hi-lo), anchorlo, anchorhi, scale)
	while (anchorhi-anchorlo)/scale < 4 do
--		print('a', anchorlo, anchorhi, scale, (anchorhi-anchorlo)/scale)
		scale = scale/2
	end
	while (anchorhi-anchorlo)/scale > 8 do
--		print('b', scale, (anchorhi-anchorlo)/scale)
		scale = scale*2
	end
--	print('c', scale, (anchorhi-anchorlo)/scale)
	while vy(anchorlo-scale) > Menu_bar_height+20 do
		anchorlo = anchorlo-scale
	end
	while vy(anchorhi+scale) < App.screen.height-20 do
		anchorhi = anchorhi+scale
	end
--	print('d', scale, (anchorhi-anchorlo)/scale)
	return anchorlo, anchorhi, scale
end