It seems like you found a bug in "library add" command. It seems there is no "build.xml" file in Chart folder. If this is the case you should run the following command from within Char folder
android update project -t "android-21" -p .
This command should generate "build.xml" in Chart folder. This is a temporary solution, just to remove the current stopper until we find the real cause of the issue.
Also, please note if there is 'src' subfolder in Chart folder. If 'src' folder is missing, just create an empty one. This folder is required by ant tool.