By choosing a proper combination of tab at the bottom of the graph, you can select a forgetting curve of interest. Vertical axis represents knowledge retention in percent.
Red curve corresponds with the best-fit forgetting curve obtained by exponential regression.
Horizontal green line corresponds with the requested forgetting index , while the vertical green line shows the moment in time in which the approximated forgetting curve intersects with the requested forgetting index line. This moment in time determines the value of the relevant R-Factor. They are followed by the number of repetition cases used to plot the graph. Note that at the beginning of the learning process, there is no repetition history and no repetition data that could be used to compute R-Factors. You can imagine that the forgetting curve graph might use average grade instead of retention on its vertical axis.