Why are temperature graphs sinusoidal? (sin graphs)?

Answer 1

What temperature graphs are you looking at?

http://www.rap.ucar.edu/weather/upper/crp.gif
http://en.wikipedia.org/wiki/Image:Co2-temperature-plot.svg

Not exactly sinusoidal. Probably the answer to you question is just the diurnal cycle of warming during the day and cooling during the night, but you won't get any good answers unless your question is more specific.