[Gnuplot-info] Trying to create audiogram chart with gnuplot - need help including custom symbols

[t p. <tee...@gm...>]

Audiograms are used in hearing tests, and they look something like the
following:

http://www.mydr.com.au/files/images/categories/hearing/audiogrammixedv2.jpg

I've tried to create an audiogram using gnuplot, but I ran into problems. It
was very easy to do the following tasks:

- set tics for x and y
- set x scale as logarithmic
- draw grid
- draw lines

However, I couldn't figure out how to include custom symbols to the chart.
The example picture shows some of the symbols that can be used. Also othe
symbols exist.

I created the symbols and stored them to PNG files, but when I used plot
command with the images I found several issues:

- Image is plotted so, that it's left upper corner is in the given
coordinates. Instead, the image's center point should be in the given
coordinates.
- Plotting images don't work with logarithmic scale (which makes sense since
they are plotted utilizing the chart's coordinates, but it isn't feasible in
this case)
- PNG transparency doesn't seem to work

What I would like to do is tell gnuplot to plot my image to specific
coordinates so that I give center point of the image and size of the plotted
image in pixels (x & y). If this is not possible can I somehow achieve
similar result?

I could even superimpose the symbols to a bitmap plot using e.g. Imagemagick
if there was a way to find out the correct position (for example, x and y
pixel locations of 70 dB & 1000 Hz).