[Gimp-user] How to transform a rectangle part to triangle?

[Kovesdi Gyorgy <kgy teledigit eu>]

Hi,

I need to transform a rectangle part of my picture to a triangle, linearly, 
only with horizontal distortion, keeping vertical positions in place. (a 
rectangle to a triangle standing on its side, by moving the top corners to one 
point)
I tried the Cage Transform: the top of the picture seems good, but it also 
distorts the bottom line in this case. Other tools seem not suitable at all.
How can I do it?

Thanx in advance
György Kövesdi