To all who replied:
I want to create a representation of the HSV cone similar to that at http://en.wikipedia.org/wiki/Image:HSV_cone.png to show how it is derived from the CIE 1931 chromaticity diagram http://en.wikipedia.org/wiki/Image:CIExy1931.svg. I want all the labels, arrows, and the wedge cut-out in this graphic -- which is unfortunately a PNG and not a vector graphic -- to be removed.
I did it in Gimp in the end as follows (thanks, Gimp listers):
*Create the base of the inverted HSV cone*
1 Use the ellipse selection to create a circle selection on a white background. (You might want to set up guides to locate the centre of the circle.)
2 Click the Blend tool, set the Gradient to Full Saturation Spectrum CCW, and set the Shape to Conical (asym).
3 Click the centre of the circular selection and draw a radius. You now have a circle filled with a coloured cone.
4 Set the foreground colour to white.
5 Change the Gradient to FG Colour to Transparent and Shape to Radial, and draw another radius. If the perimeter is not quite saturated enough, copy the FG Colour to Transparent gradient and adjust the copy so that the transparency sets in earlier.
6 Change the proportions so the circle becomes an ellipse, longer in its horizontal axis. This is the base of the inverted cone.
7 Add an alpha channel to the ellipse layer.
8 Change the foreground colour to black, then use the Bucket Fill tool to change the four white corners of the ellipse to black.
9 Choose Select > By Colour, click the black, and cut the black selection.
*Create the cone*
The cone will make a tangent to the ellipse a little around the bottom curve from the ends. So you will first need to copy the Full Saturation Spectrum CCW gradient and adjust the copy's end colours to match those at the point of the tangents.
1 Create a new document with a square selection on a white background.
2 Change the blend tool to Gradient: Custom copy of Full Saturation Spectrum CCW.
3 Drag the blend tool horizontally from the L to R side of the square.
4 Press Ctrl+a to select the entire square.
5 Click the Perspective tool.
6 Drag the bottom L and R handles to the vertical centre line, keeping the horizontal grid line horizontal. You don't need to exactly place the L and R corners at the centre, as this is fixed later, as long as the general effect of the bands of colour fanning out from the point on the bottom edge to the full width on the top edge is achieved.
7 Set the foreground colour to black.
8 Click the Blend tool and set Gradient to FG Colour to Transparent, and draw from the "point" bottom edge to the centre of the full width top edge.
9 Click the Bezier tool and draw an inverted triangle from the top L corner to the bottom centre, the top R corner and close the curve.
10 Change the path to a selection and copy the selection.
11 Make a new layer under your ellipse in the first document, paste your inverted coloured triangle into that layer, and move it around until the sloping sides are tangents to the ellipse.
The colours in the inverted triangle don't quite match up with those along the bottom edge of the ellipse but the effect of the HSV inverted cone is good enough to fool a fast moving observer.
Regards, Hedley