What values to be taken for x-axis and y-axis?
It doesn't matter really, but for aesthetics sake, you'd plot the wavelength on x and the absorbance on y. We tend to visualize such data as spectral or absorbance as "peaks" on the range of interest. There's a fairly good example here. It has something to do with convention: traditionally 
 the independent variable on x and the dependent variable on y. There's a good answer on cross-validated on the matter, too.