What's the difference between a wavelength and an an amplitude?

Wavelength - the distance from one wave crest or trough to another wave crest or trough. Amplitude - the distance from the median point or "middle" of the wave straight up to a crest (a maximum) or straight down to a trough (or minimum), which is the peak amplitude; or the distance from a trough straight up to a crest, or a crest straight down to a trough, called peak-to-peak amplitude.

A wave is propagated by a point or a particle in a medium that vibrates or oscillates between its mean position and a maximum displacement position.

Amplitude is the maximum displacement from the mean position.

Due to the vibration of the particle, neighbouring particles get disturbed and they also oscillate but some small time (phase) behind.

Along the direction of propagation of the wave, the wave travels a wavelength distance in the same time that is taken by a particle to complete one oscillation.
It is the distance between two successive crests (maximum displacement positions - When a snap shot of wave is taken at one point of time.

Alinear relationship would look like a straight line on the graph. it could have a positive negative or 0 slope. so it could be a line potentially going in any direction but it must be perfectly straight.