Is abnormal left axis deviation serious

f a line is drawn parallel to the angle of incidence axis (X-axis), it cuts the graph at two points, showing that there are two values of angle of incidence for an angle of deviation. However, at the point of angle of minimum deviation, the line will be tangent to the curve showing that for minimum angle of deviation there is only one angle of incidence.

Amplitude is the amount of deviation of a wave from the mean position , measured in Y axis when the sound waves are moving along the X axis.

Yes, the x axis is left to right.

Look at the Wikipedia article on "Standard deviation" - it includes an example right at the beginning.