For a filament lamp, the graph curves because as the filament heats up, its resistence heats up too. As voltage goes up, current decreases.
For a resistor current is directly proportional to the voltage.
Since current is only allowed in one direction through a diode, the graph will only be active on either the positive or negative side.
If voltage increased enough, the diode conducts in other direction (Break Down Voltage).
Resistance decreases as temperature increases.
LDR – Light Dependant Resistor
Resistance decreases as light intensity increases.