Add these lines to the bottom of your code, to plot the desired value as a red asterisk on the existing plot with the other data plotted. Then reference the plot handle "p_include", from a legend request, to get a legend with only this plot data value included. string( R) converts R to a string and assigns it as a name for the plot which plots it on the figure.
p_include = plot(C,R,'r*','DisplayName',string(R))
chances are, your added plot of C,R was being plotted as a tiny line segment, but you just couldn't see it on your plot since it only had 1 data point.
The red asterisk will make the data point show up more clearly on your plot, and the legend will contain the numerical value R of the max value
I hope this helps ! :-)