As you go from left to right, you are increasing the amount of Al₂O₃ in the mixture, and the corresponding percentage is the mole fraction (number of moles of Al₂O₃ divided by the total number of moles) of the mixture. On the x axis, we have the mole % of Al₂O₃ in the mixture.
Here is an example of a phase diagram for an SiO₂ and Al₂O₃ mixture.
