Previously these were set to -1, but the test for colouring was just if value > -1.
So all negative values were coloured "invalid".

This changes the flag value to be negative double::max, and make the test equal.
This does not affect negative values (apart from -Double::max, which must be v rare).

This also uses a common constant rather than several magic -1's.

Also checks if the integrated range value is NaN (because there were no valid values, and will make that invalid).

Also removed a commented out Parrallelisation clause.  No explanation why it was commented.
I went back through blame and it only seemed to appear already commented.
Removed to save anyone else the confusion I suffered.