rgbimage plots with linked axes

[Dima K. <gn...@di...>]

Hi. I just stumbled on another bug with rgbimages. If somebody knows
what's causing it, that'd be appreciated.

I'm plotting an image showing a map composed of stitched openstreetmap
tiles (attached). The mapping between pixels in this image and lat/lon
is linear in longitude and nonlinear in latitude. I'm plotting the image
pixels onto x2y2, and using linked axes to get the latitude on y and
longitude on x. The script is this:

  set x2tics auto
  set y2tics auto

  lon0       = -123.5614
  lon_offset = 180.0 + lon0

  zoom = 4.
  px_to_lon(x) = x/(256. * 2.**4)*360. - 180. + lon_offset
  lon_to_px(x) = (x-lon_offset+180.)/360. * 2.**zoom * 256.

  lat_offset_px = 1215.969

  s1_cos(x)     = (sin(x)+1)/cos(x)
  inv_s1_cos(x) = asin( (x**2 - 1)/(x**2 + 1) )

  px_to_lat(x) = inv_s1_cos(exp((1 - (x + lat_offset_px)/(2.**zoom * 256) *2)*pi))*180./pi
  lat_to_px(x) = (1 - log( (sin(x*pi/180) + 1.)/cos(x*pi/180) )/pi)/2. * 2.**zoom * 256 - lat_offset_px


  set link x2 via lon_to_px(x) inverse px_to_lon(x)
  set link y2 via lat_to_px(y) inverse px_to_lat(y)

  plot [x=-120:-80][y=20:60] "montage_40_-99_1300miles_4.png" binary filetype=png flipy with rgbimage notitle axis x2y2, '-' with points axis x2y2
  60 419
  e


Note that the circle representing Los Angeles lies at pixel coordinates
60,419 and I'm plotting a point there. The corresponding latlon is
roughly -118,34. The pixels are on x2y2 and the point is on x2y2, so
this point should be rendered in that circle. I'm not seeing that at
all, however. The point is at y2=419 as it should be, but the Los
Angeles circle is roughly at y2=448. The x coordinates match properly.

Furthermore, zooming in makes it clear that gnuplot is confused:
interactively zooming on the plotted point shows me the zoomed LA
circle, even though visually the LA circle was elsewhere.

Any pointers?