Hi Jan-Hendrik,
I browsed through the properties of graph, but could not find a way to change the marker grid width either.
Time ago I had the same need, what I did was to add flat plots of constant value (lines) to the chart and customize their appearance as plots.
If you want to place these lines progammatically based on the position of your markers, you should use the property Y-Scale.MarkerValues that returns an array of matker values of the selected scale; use these values to build arrays to be displayed on your chart.
Hope I've been clear enough, let me know,
Alberto