Should be a simple graph - but!!!

A typical plot that I just can't make work would be for the following simple
data for various aircraft:

X-Axis values (cruise speed - mph) - 315 325 355 520 and
555
Corresponding Y-Axis Values (mpg) - 0.4 1.11 0.52 0.25
and 0.11

Couple of questions:

1) How do I get even increments of cruise speed on the x-axis, say 0 to
600, in even 100 increments, with data points properly spaced?

2) What is the real difference between Chart Options of Category and
Automatic?

Dave