The applet above calculates and plots the output characteristics of an n-channel (enhancement-mode) MOSFET. Try to change the drain-source voltage (Vds) range and/or the gate bias starting value ('begin') or other values and see the drain current vs. drain bias (Vds) change.
Since the axes re-scale themselves according to the maximum x- and y- vaules calculated, the curve shapes may appear varying inconsistently. The text area after the "function:" label shows the actual function that was coded to calculate the drain current.