I'm reading an article on MOSFET based amplifiers and saw the next figure:
I think this figure is incorrect as you can't draw IDS without knowing both VGS and VDS.
Moreover, IDS is always dependant on VGS, so it doesn’t remain constant as VGS goes up.
It's not incorrect: it's merely showing you the curve for one particular (unspecified) value of Vds. You'd get similar shaped curves (albeit with different slopes and plateaux) for other values of Vds.
Moreover, IDS is always dependant on VGS, so it doesn’t remain constant as VGS goes up.
Not so. The curve shows that Ids increases with Vgs up to a certain point. Beyond that point there's a plateau, where Ids is almost constant as Vgs increases.
I don't see anything wrong with the device curve (Ids vs Vgs), but the "load line" doesn't make sense. Class A operation does require biasing somewhere near the middle of the curve, but I don't know what the straight line represents.