In a speech and audio processing class the lecturer pointed out that the vowel space is indicated by F1 plotted against F2-F1.
I have searched online, and most sources I've found (including this paper Sharifzadeh, H. R., McLoughlin, I. V. and Russell, M. J. (2012) ‘A Comprehensive Vowel Space for Whispered Speech’, Journal of Voice, 26(2), pp. e49–e56) just mention that that is how the plot is traditionally made, but I can't seem to find the reason.
I will of course ask him if I can't find an answer by the next class.