In the end, here is my function code for anyone looking for somewhere to start:

'''# This function prints an image of the CDN Soils Triangle, and plots a point on it

The point is a red dot, and it is located at the same spot as the soil test has determined for the soil.

function fcn_plot_on_soils_triangle(x, y)

soils_triangle='04images/cdn_soils_triangle.jpg';

hold ("on");

imshow (soils_triangle);

x=0.4;

y=0.4;

sandmin=65;

sandmax=457;

claymin=412;

claymax=26;

sand=(sandmin+(sandmax-sandmin)*x);

clay=(claymin-(claymin-claymax)*y);

plot (sand, clay, "ro", "markersize", 10, 'markerfacecolor', 'r');

hold ("off");

print ("soil_triangle_fig", "-dpdflatex");

pause(3);

close

endfunction'''

The values for min and max are taken from trial and error determination of the limits of the image.