When plotting the motor noise graph, keep the following in mind.
BVE works by taking a sample of the motor noise, and bending the pitch and changing the tone in accordance to what you put into the train.dat file to produce the motor noises of the train. A good sample of a motor noise for use in BVE should be about a second or more of consistent pitch and tone.
To plot the graph-
x-axis = speed of the train in which a sound would play at a specific tone/pitch
y-axis = pitch that the motor sound should be when the train is at that specific speed labeled in the x-axis. 100% pitch = play original motor.wav file.
When plotting volume, the x-axis serves the same purpose while the y-axis serves as how loud the sound will be played. In most trains, as the trains travel at a faster speed, the motor noises begin to fade (thus making this feature needed)
The rest is all trial and error..when you've been doing this for almost 6 years you kinda get the hang of it lol