Hi Keith,
You need to fit what you want within the bounds of the graph type you are using.
a) Line graph has a dynamic Y axis, but a discrete X-Axis - the x-Axis contains "point numbers" which are integers starting from 1.
It seems to me you're setting the point number to be RMP/1000, so 1600 becomes 1.6 which is not an integer.
I would probably drop the x 1000 part, and just use the RPM value as the point-number "as is".
cheers
Bruce