As a proof of concept I have made a small patch to the graph panel code that allows control of y axis tick annotation and grid line placement such that graphs such as these can be generated.
The configuration for the first one is:
The annotations are defined by providing an array of values as can be seen. If an element is a number then it is placed on the grid at the appropriate position. If the element is itself an array then the first element is a number that determines the position on the axis. The second element is a string which is shown at that position. A grid line is drawn for each provided value on the left axis. The range of values does not have to match the y-min and y-max values, which can be auto if required. The annotations are placed at the correct positions to match the data. Any annotations off the top or bottom of the chart are ignored.
Whether this is of interest to the community I do not know. Personally I find it extremely useful. If anyone wants to have a look at the code it can be found on the graphpanel-y-ticks branch of my github fork of grafana. It involves small changes to just two files.