Supporting Trend Arrows

[hexapode]

Hi!

I suggest we add a support for 'trend' arrow in the format.

Should it be in an arrows sub object?

 {
   arrows : [
       from : "nodeId",
       maturity : .5,
       visibility : .5
   ],
  ...
}

[cioportfolio]

Sorry for neglecting this comment for so long. Happy to merge in a pull request if you prepare the changes but you may have moved on to other things after this long wait!