It should look something like this:

It mean the colder your solar panel gets, the higher the voltage of the panel goes. The easiest way to estimate how much a solar panels voltage will increase is to refer to the following code: NEC Article 690 Table 690.7(A) Voltage Correction Factors for Crystalline and Multicrystalline Silicon Module The last entry of that table means if the solar panel lives in a place where the tempurature can range from -32F to -40F you use "1.25" as the factor. So if you have a solar panel with a VOC (Open Circuit Voltage) of 40 volt. You times that 40 volts by 1.25, giving 50 volts. Which means if the solar panels tempurature is -32F, and gets enough sunlight energy, it will produce to 50 volts and not 40 volts. In short, make sure that the MPPT charger you buy has a high enough PV voltage rating to handle your solar panels increasing their voltage by 25% here in Idaho.