Does anyone know how to apply custom marker labels to a chart?

I have a chart, plotting newspaper circulation over the course of a few years. A few times during that time period our cover price changed and I need to label the chart with those changes.

My graph has markers already but they're irrelevant to the pricing changes and there are too many of them.

I want to reduce the number of markers along the graph to include only the time the pricing changed, and label the markers accordingly.


Is this something I can do?