The XY axes in the first chart are F1-F2 formants, which are related to the frequency of the vowels. The XY axes of the 2nd chart correspond to the XY position of the tongue in the mouth and it is thus much more useful to language learners. I recommend that you use that one for learning French vowels.
Wikipedia has also a chart with all vowels in IPA, in the same format of the 2nd chart: en.m.wikipedia.org/wiki/IPA_vowel_chart_with_audio
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