when used as a mutable data-structure. Plain std::vector halves the time
taken to display the chart dialog

Create a class to represent the std::vector we are going to be passing
around, and move some of the utility methods into it to make the code
prettier.
Also create an optimised append(&&) method for the common case of
appending small temporaries.

Change-Id: I7f5b43fb4a8a84e40e6a52fcb7e9f974091b4485