All the previous answers say that the voltage from a thermocouple is a function of the temperature difference between the hot and cold junctions. This is a somewhat crude approximation, and is not entirely true.
For example, if the hot junction is at 100°C and the cold junction is at 0°C, the voltage will not be exactly the same as if the hot junction is at 200°C and the cold junction is at 100°C. For a type K thermocouple, the former will be 4.096mV and the latter 4.042mV. The difference may appear to be small (about a 1% error), but it's not negligible in many cases.
By convention, thermocouple voltages are given with a cold junction temperature of 0°C (because it is easy to create a constant accurate temperature of 0°C/32°F with an ice bath). Now, the voltage from a thermocouple with both ends at 0°C must be 0uV (should be obvious from thermodynamics/conservation of energy), so the graphs will always pass through 0 (when 0 is selected as the cold-junction temperature!). Graphs made with °F usually pass through 32, sometimes 75°F.