Steps to reproduce the problem:
1. When using the demo application - https://mozilla.github.io/pdf.js/web/viewer.html, load  the document 5311693-1.pdf

[5311693-1.pdf](https://github.com/mozilla/pdf.js/files/5137225/5311693-1.pdf)

2. go to Page 16 of the document.

3.  The strip of  EKG data  appears to have an incorrect line width   under the  Multi-color heat rate chart.

4.  When the same document is generated by pdffactory ,  the artifact are not visible.

What is the expected behavior? (add screenshot)
![expected](https://user-images.githubusercontent.com/3577617/91461157-4622cb80-e84e-11ea-8e5d-750c7b4e64ae.PNG)

What went wrong? (add screenshot)
The EKG line strip  has an incorrect thickness.  the x-transform seems to stretch the line thickness.  See the  stri in the RED rectagngle of the attached behavior versus the GREEN rectangle

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):
 https://mozilla.github.io/pdf.js/web/viewer.html
ctx.restore();
      ctx.clip();
      ctx.beginPath();
      delete this.pendingTextPaths;
    },
    setCharSpacing: function CanvasGraphics_setCharSpacing(spacing) {
      this.current.charSpacing = spacing;
    },
    setWordSpacing: function CanvasGraphics_setWordSpacing(spacing) {
      this.current.wordSpacing = spacing;
    },
    setHScale: function CanvasGraphics_setHScale(scale) {
      this.current.textHScale = scale / 100;
    },
    setLeading: function CanvasGraphics_setLeading(leading) {
      this.current.leading = -leading;
    },
    setFont: function CanvasGraphics_setFont(fontRefName, size) {
      const fontObj = this.commonObjs.get(fontRefName);
      const current = this.current;
      if (!fontObj) {
        throw new Error(`Can't find font for ${fontRefName}`);
      }
      current.fontMatrix = fontObj.fontMatrix
        ? fontObj.fontMatrix
        : FONT_IDENTITY_MATRIX;
      
      
      if (current.fontMatrix[0] === 0 || current.fontMatrix[3] === 0) {
        warn("Invalid font matrix for font " + fontRefName);