Charts are basically just charts you can configure to be added to dashboards from a variety of sources in your Dynatrace deployment and you have quite a bit of control over the data being used and how it is displayed: https://help.dynatrace.com/get-started/dashboards/...
Reports (Service Quality and Availability reports) are more "hard-coded" at the moment and are pre designed and created I believe at Midnight on Sundays that summarize data over the past week. Here is some info on service quality reports: https://help.dynatrace.com/get-started/reports/wha... they give you scores and details about various areas of the site/application.
The availability reports are based on web checks that you have running in Dynatrace. The web checks are the only thing that have an additional cost, the actual running and viewing of the reports do not.
Charts can use metrics about services, applications, processes, or hosts. You select the data you want to see in what format is useful for you.
The reports are simplified summaries that will give you an idea of how your application performed over the last week. It is almost like an executive summary that would be good for people who don't need to drill into or build their own charts and dashboards.
Yes, they are based of the same data but charts let you pick exactly what you are seeing and they are for different use cases. A report for a high level look at performance vs charts and dashboards for lower level or more detailed looks for monitoring or analysis of an issue.
From that linked documentation:
"Reports are structured in such a way that even non-technical team members can understand them."
Have you had the chance to look into the generated reports and maybe make some custom charts to more easily see the difference?