I just wanted to reach out and let you know that we do offer a text search/filter on the Crash Reporting dashboard located in the upper portion of the left side navigation bar. I do understand what you mean by wanting to track an individual error group over time. While the main dashboard timeline shows all exceptions/errors, you can get a partial time line on specific error groups when you click on them in the dashboard to go into the error details. This information is in the right sidebar and gives background on the exception over time.
While this may not be exactly what you're looking for, I can say that we are working hard on custom reports that may enable you to get a much clearer graphical representation of that single error group data over time.
Please let us know if you have any other questions or feedback!
Jesse, Developer Evangelist at Raygun