Include frequency values when saving chart data from average plots

[David Platten]

I would find it useful to include the frequency as well as the mean or median value when saving chart data to a csv or xls file.

[David Platten]

Altered chart saving to csv and xls to include frequency data where it is available. Fixes issue #360

→ <<cset 7489266ae34f>>

[David Platten]

Updated changes files. References issue #360

→ <<cset efe52599640d>>

[Ed McDonagh]

Merging chart simplification branch. Great job @dplatten! I'll test it properly after merging it in. Refs #351, #360

→ <<cset 138e952ea942>>

[David Platten]

@edmcdonagh, I've just totted up the added and removed lines from this branch:

Added: 2401 lines Removed: 6787 lines

A net reduction of 4386 lines of code...