Dashboard charts are based on a document type?

This is the first time I am creating a dashboard chart and I am having difficulties accessing the correct data to create it.

Basic question, all the data that I need to access needs to be in one document type, correct?

If so, if the variables are in different document types I should create a new doctype and add those variables, correct? If this is the case, is there a guide on creating a document type, I tried adding variables from different doctypes but it did not work.

If my line of thinking in incorrect, feel free to guide on a better solution. Thank you!