its quite simple. First make sure that the data is continuous which in your question is i.e when ir say 10<x<15, the next one shud also start with 15 i.e 15<x<25. Then make sure that the class width is equal i.e 15-10 or watever. If class width is equal u plot the frequency and if not then u plot the frequency density which is
Frequency Density=Frequency divided by class width
