Taking a look at the I-V graph

The graph typically has potential difference (V) on the x-axis and current (I) on the y-axis.
If the graph is a straight line passing through the origin, the component obeys Ohm's Law. If it's a curve, the opposite is true.
The gradient of the I-V graph at any point represents the reciprocal of the resistance (1/R) of the component at that particular voltage.
A steeper gradient means lower resistance, a shallower gradient means higher resistance.

I-V characteristic of a filament lamp

A semiconductor diode allows current to flow in one direction only.
The I-V characteristic for a filament lamp is a curve that starts with a steep gradient at low voltages and flattens out as the voltage increases.
As the voltage and current increase, the filament heats up, causing the resistance to increase.
The increasing resistance causes the gradient of the I-V curve to decrease.

The I-V characteristic of a diode shows a very low current at negative voltages (reverse bias), up to a point where the current starts to increase very slightly.
At positive voltages (forward bias), the current remains very low until a certain voltage, the threshold voltage, is breached, after which the current increases rapidly.
The diode has a very high resistance in reverse-bias and a very low resistance in forward-bias when the threshold voltage is breached.

When a metallic conductor is Ohmic, the I-V graph shows a straight line passing through the origin.
This means that resistance is constant and the conductor obeys Ohm's Law.
However, changes in temperature and pressure may cause the resistance to change, changing the gradient of the I-V graph.

The I-V characteristic of an NTC thermistor is a straight line (Ohmic) when the temperature remains constant.
However, the resistance decreases as the temperature increases, causing the gradient of the I-V graph to increase.
Many thermometers use the resistance of an NTC thermistor to determine the temperature of the environment.