Plotters are just one small step removed from a 2D mill. This particular kind of plotter, where the paper is fed through to obtain one axis of movement suffers from bad registration, any kind of slippage on the paper (and there always will be some) will lead to registration errors. Try different kinds of paper to determine which kind gives you the least slippage by repeatedly drawing very large patterns in different colors and then studying the horizontal edges near the limits.

The command set looks like a modified HPGL.