Fix category display in Google Slides

[christiankiely]

This fixes an issue when importing generated PPTX files with charts into Google Slides.

It appears Google Slides' importer does not like the use of multiLvlStrRef, and displays categories as simple numbers.

---

For example, using this code to generate a pptx file:

  const pres = new pptxgen();
  const slide = pres.addSlide();
  slide.addChart(
    pres.ChartType.bar,
    [
      {
        name: "Categories",
        labels: ["Category A", "Category B", "Category C"],
        values: [500, 200, 350],
      },
    ],
    {
      x: 1,
      y: 1,
      w: 5,
      h: 3,
    }
  );

Appears like this in PowerPoint vs Google Slides:

Microsoft PowerPoint	Google Slides

---

Using strRef instead of multiLvlStrRef when there is only one set of labels appears to fix this issue.

• Example pptx before this change
• Example pptx after this change