In general, from a MCA, I expect data in form “channel number” plus its “number of counts”. In ROOT, you can easily read files which contain two columns of numbers and create a TGraph. Moreover if you want to “calibrate” your “channel numbers” into a quantity like “pulse-height” or “time” or “energy”, you will often need to apply some nonlinear function, which will again be easy for a TGraph but, with a histogram, you would need to use a “variable bin sizes” histogram (and this may not be that straightforward, if you started with a “fixed bin sizes” histogram).