code: function toStepping ( xVal, xPct, value ) {
  
          if ( value >= xVal.slice(-1)[0] ){
              return 100;
          }
  
          var j = getJ( value, xVal ), va, vb, pa, pb;
  
          va = xVal[j-1];
          vb = xVal[j];
          pa = xPct[j-1];
          pb = xPct[j];
  
          return pa + (toPercentage([va, vb], value) / subRangeRatio (pa, pb));
      }
docs: (percentage) Input a value, find where, on a scale of 0-100, it applies.