Base calling is the process of assigning nucleobases to chromatogram peaks. The difference appears as a peak in the chromatogram. There is therefore always a background signal present in the chromatogram. The increase in current is translated and appears as a peak in a chromatogram. The results are then visually displayed in a resulting chromatogram. A simplified method of calculating chromatogram resolution is to use the plate model. The resolution can be calculated from the chromatogram. The chalk chromatogram, with separation of colours, can then be observed in a dark room. This can give rise to ghost peaks in the chromatogram. Certain types of mass spectrometry data are best represented as a mass chromatogram. The ion chromatogram shows a typical chromatogram obtained with an anion exchange column. In this case, the x-axis represents retention time, analogous to any other chromatogram. Development of the chromatogram is done by allowing the solvent to travel down the paper. Since only fluorescence is measured, only the tagged protein is seen in the chromatogram. This plot of sample concentration versus time is called a chromatogram.