function isCdIndexValid(v) {
        return v !== undefined && v < calcTrace.length;
    }
Tag selected calcdata items

N.B. note that point 'index' corresponds to input data array index
 whereas 'number' is its post-transform version.

@param {array} calcTrace
@param {object} trace
 - selectedpoints {array}
 - _indexToPoints {object}
@param {ptNumber2cdIndex} ptNumber2cdIndex (optional)
 optional map object for trace types that do not have 1-to-1 point number to
 calcdata item index correspondence (e.g. histogram)