message: Modify secondary axis param check.  Only throw if number of valAxes is greater than 1.
diff: @@ -499,7 +499,7 @@ export function makeXmlCharts(rel: ISlideRelChart): string {
  // B: Axes -----------------------------------------------------------
  if (rel.opts._type !== CHART_TYPE.PIE && rel.opts._type !== CHART_TYPE.DOUGHNUT) {
  // Param check
  - if (rel.opts.valAxes && !usesSecondaryValAxis) {
  + if (rel.opts.valAxes && rel.opts.valAxes.length > 1 && !usesSecondaryValAxis) {
  throw new Error('Secondary axis must be used by one of the multiple charts')
  }
  