What is the shape of the I/V graph for a filament lamp?

Starts at 0,0 then curve with a positive gradient, whose gradient decreases and the I and V increase.

It looks like this!

You don't strictly need the negative quadrants, but there's nothing to stop you applying a negative voltage to a filament lamp, it would just drive the current in the opposite direction, so the graph is rotationally symmetrical.

It is always a straight line.

A S shape curve, as potential difference increases, current increases at a decreasing rate.