Why graphit to diamon is endothermic

Hi,

I need some help to found the mistake in my reasoning :

I understand that the reaction carbon to diamon is endothermic. But, if i look the phase diagram of crabon the slope of the equilibrium curve for this transformation is positive. So dP/dT > 0. Clapeyron :

 

Moreover Vdiamon < Vgraphite so  to have a positive slope we need have Delta H transition negative, this is caracteristic of exothermic reaction, no ?

Where is the mistake on my reasoning ?

Thank you.

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