An I-V plot of a resistor is a straight line, often referred to as a linear I-V plot; that is, each successive and equal increment of voltage will produce an equal, incremental increase in current. A I-V plot is often modeled mathematically as a linear function; meaning that the current through a resistor is linearly proportional to the voltage across it.
An I-V plot of a LED bulb is exponential; meaning that each successive, equal increment in voltage will produce substantially greater amounts of current. The I-V plot of an LED bulb is modeled mathematically as an exponential function; in effect, the current through the LED is exponentially proportional to the voltage across it.
From an I-V plot, you can find the resistance of an electronic component at a specific voltage level. To do this, select a voltage level and find the associated current level on the I-V plot. Then divide the voltage by the current.
For a resistor I-V plot, this calculated value, the resistance, will always be the same regardless of the voltage chosen. However, for an LED bulb, the resistance calculated will never be the same. The higher the voltage applied across the LED bulb, the lower the resistance of the bulb.
Different resistors will have I-V plots that are lines but have different slopes. A small resistor, such as 10 ohm resistor, will have an I-V plot with a slope that is much higher than the slope of a 100,000 ohm resistor.
The I-V plot will also differ for different LEDs. LEDs that can produce a very bright light will have I-V plots that are steeper than LEDs that can't produce as bright of a light. And that's because brighter LEDs produce a higher current at a given voltage than a brightness-limited LED can.
If you have an LED design that requires that all the LED bulbs produce an equivalent amount of light, you will have to ensure that their individual I-V plots are as similar as possible. Because manufactures don't guarantee that different LEDs will have the same I-V plot, to find out how similar I-V plots of different LEDs are, you will have to make your own.
You need a power supply, a voltmeter and an ammeter to make your LED's I-V plot. Apply different voltages across the LED with your power supply and measure the current through the LED for each different voltage. Plot each of these voltage/current readings on graph paper. Use the vertical axis for current and the horizontal axis for voltage. Connect the points together to make the I-V plot.
If your make I-V plots of several different types of LED bulbs you might find that some LED bulbs don't have I-V plots that you would expect. And that's because these LED lights have other electronic circuits inside them. So when you construct the I-V plots of these devices, you are not constructing an IV plot of an LED but an electric circuit with an LED within it.