The frequency is not directly proportional to the wavelength - it's inversely proportional: https://en.wikipedia.org/wiki/Proportionality_(mathematics)#Inverse_proportionality
Think of this as this: The wavelength is the distance that light travels during one wave i.e. cycle. Light propagates with the speed of light, so the smaller the wavelength, it means the frequency must increase. If the wavelength gets two times lower, the frequency increases two times. If wavelength approaches 0, then frequency starts growing very quickly, approaching infinity.
The plot is not a straight line but a hyperbola.
wow TIL sth as well I guess