Just as you said, the chart displays as a static image after exporting to excel, and it cannot be edit. The feature that a chart can be editable after exporting to Excel is not supported in current release of Reporting Services. One workaround for you is
create a chart base on the same data in the excel level. Notice that there are many features supported on one side but not the other.
Alternatively, if you have any concern about this, please submit a feedback at
https://connect.microsoft.com/SQLServer/Feedback. Connect site is a connection point between you and Microsoft, and ultimately the larger community. Your feedback enables Microsoft to make software
and services the best that they can be, and you can learn about and contribute to exciting projects.
Microsoft is conducting an online survey to understand your opinion of the Technet Web site. If you choose to participate, the online survey will be presented to you when you leave the Technet Web site.