The X axis is the track data starting at left edge, and ending at the right. The Y axis is the density of data at any position (X) on the track.
The black line in the middle is the theoretical track optimum (but this is impossible to achieve with an analogue device). The graphs visualise the +/- 15% density space, the upper area (above optimum) is higher density, and the lower (below optimum) is lower density.
The actual original values measured by the dumping software are shown in the background in pink. The “trend” density is shown in blue, it is a mathematical representation of the original and is quite useful to automate track type recognition.
These examples serve to illustrate how interesting some track density graphs are.
Above is a normal track, as written without any special density tricks. You may notice that the track data shows fuzziness, but this is normal, it is just how the device works.
The above example shows a Commodore Amiga Copylock track, which is about around 6-7% higher density before middle block, the middle block slightly off normal, and after the middle block around 6-7% lower density. The last few bytes of this track have actually not been written, as shown by the sudden vertical line at the end.
This Long Track contains about 6% more data than a normal track would.
This long track with partially unwritten data is quite interesting. Noise cannot be detected properly without mathematical analysis, and it clearly shows that often, the long track capacity is only used to fool copiers. Removing the noise in this example means that the data can fit perfectly into a normal track. This means of course that this type of protection is remasterable if the noise is removed.