How do I calibrate a plot axis from channel numbers to energy?

Using a gamma ray spectrometer I took a spectra of a sample and I want to be able to plot the results. When I plot the data, the x-axis is the channel numbers (4096 channels). Each of the Channels corresponds to a given energy. If I had a function to convert from channels to energy it would be great. However what I do have is a list of 8 calibrations (i.e. Channel x corresponds to energy y). How do I calibrate the axis using these points?