The y values are scaled so they will range between 0.5 and 0.95 for all the data.
The first step was to use the gear menu shortcut (click the gear menu on Values > Mapping shortcuts > Scale [0,1]. The additional factor of 0.9 and adding 0.5 in the equation shifts the scaling between [0.5, 0.95]. You don’t necessarily need to do this but then the lines do not touch the boundaries.
By scaling to these values, the scaled values are all added an integer value corresponding to the Sequence of the Category. Notice if you expand the Category column definition, the Values menu is set to “Sequence #”, where A = 1, B = 2, C = 3.
That shifts the values up an increment of one for each Category on the y-axis.