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
this.transparentCanvas = null;
      }
      this.cachedCanvases.clear();
      this.webGLContext.clear();
      if (this.imageLayer) {
        this.imageLayer.endLayout();
      }
    },
    
    setLineWidth: function CanvasGraphics_setLineWidth(width) {
      this.current.lineWidth = width;
      this.ctx.lineWidth = width;
    },
    setLineCap: function CanvasGraphics_setLineCap(style) {
      this.ctx.lineCap = LINE_CAP_STYLES[style];
    },
    setLineJoin: function CanvasGraphics_setLineJoin(style) {
      this.ctx.lineJoin = LINE_JOIN_STYLES[style];
    },
    setMiterLimit: function CanvasGraphics_setMiterLimit(limit) {
      this.ctx.miterLimit = limit;
    },
    setDash: function CanvasGraphics_setDash(dashArray, dashPhase) {
      const ctx = this.ctx;
      if (ctx.setLineDash !== undefined) {
        ctx.setLineDash(dashArray);
        ctx.lineDashOffset = dashPhase;
      }
    },
    setRenderingIntent(intent) {