I did'nt find a Cepheid in my constellation so I've to look for one in sagitta constellation (S sagittae):
I don't know how to get an image of the Cepheid's period and luminosity, but I know how to calculate the apparent magnitude, and I know the process to make the graph of the period and the luminosity in excel.
The period/luminosity graph
You have to take the column that have the luminosity and the period and then put it in the excel columns, then you have to choise the graphic option.
To calculate the magnitude you have to take the m aparent - m absolut