This graph shows that an increase in altitude causes a relative decrease in boiling point temperatures. The two variables, it can be said, are inversely proportional.
This is because, with an increase in altitude, there is a proportionate decrease in air pressure. This means the vapor pressure of the fluid becomes strong enough, at a relatively lower temperature, to overcome the air pressure and for the liquid to boil.
This is why water can even boil without inputting heat into it but rather by just reducing the ambient air pressure.
boiling point is the temp at which a liquid's vapor pressure equals the external pressure. At higher altitudes the external pressure is lower so vapor pressure becomes equal to external pressure at a lower temperature which is why boiling point decreases with increasing altitude.
