Is there a correct and nice way to tell relatives that can't afford to send a graduation gift not to send anything that you just wanted to let them know your daughter is graduating?

How very kind you are to realize this in your relatives. If the relatives live in another town or country then simply say that there will be a celebration party at your home for your daughter's graduation with her friends only and the request is 'no gifts' stipulated by your daughter. However, if the relatives live within the vicinity then I wouldn't say anything. People give what they can and you can't strip them of that dignity. They wouldn't do it if they didn't want too.

You could go shopping yourself and look at some inexpensive jewelry and choose something your daughter may like and tell them what store it is at and your daughter loves it, but you refused to buy it for her. This way they are giving a gift without spending too much money. We call them 'KiOSks' in Canada, but they are small jewelry stands in the mall where you can purchase some lovely jewelry that isn't expensive.